Gas separation using C3+ hydrocarbon-resistant membranes

ABSTRACT

A process for separating a gas from a gas mixture containing the gas and a C 3+  hydrocarbon vapor, using gas-separation membranes selective for the gas over the C 3+  hydrocarbon vapor. The membranes use a selective layer made from a polymer having repeating units of a fluorinated polymer, and demonstrate good resistance to plasticization by the organic components in the gas mixture under treatment, and good recovery after exposure to liquid aromatic hydrocarbons.

FIELD OF THE INVENTION

The invention relates to the separation of gases from hydrocarbon gasmixtures, such separations including hydrogen from hydrocarbons, carbondioxide from hydrocarbons, and hydrocarbons from one another. Theseparation is carried out using hydrocarbon-resistant membranes, and isuseful in refineries, petrochemical plants, natural gas fields and thelike.

BACKGROUND OF THE INVENTION

Polymeric gas-separation membranes are well known and are in use in suchareas as production of oxygen-enriched air, production of nitrogen fromair, separation of carbon dioxide from methane, hydrogen recovery fromvarious gas mixtures and removal of organic vapors from air or nitrogen.

The preferred membrane for use in any gas-separation applicationcombines high selectivity with high flux. Thus, the membrane-makingindustry has engaged in an ongoing quest for polymers and membranes withimproved selectivity/flux performance. Many polymeric materials areknown that offer intrinsically attractive properties. That is, when thepermeation performance of a small film of the material is measured underlaboratory conditions, using pure gas samples and operating at modesttemperature and pressure conditions, the film exhibits high permeabilityfor some pure gases and low permeability for others, suggesting usefulseparation capability.

Unfortunately, gas separation in an industrial plant is seldom sosimple. The gas mixtures to which the separation membranes are exposedmay be hot, contaminated with solid or liquid particles, or at highpressure, may fluctuate in composition or flow rate or, more likely, mayexhibit several of these features. Even in the most straightforwardsituation possible, where the gas stream to be separated is atwo-component mix, uncontaminated by other components, at ambienttemperature and moderate pressure, one component may interact with themembrane in such a way as to change the permeation characteristics ofthe other component, so that the separation factor or selectivitysuggested by the pure gas measurements cannot be achieved. In gasmixtures that contain condensable components, it is frequently, althoughnot always, the case that the mixed gas selectivity is lower, and attimes considerably lower, than the ideal selectivity. The condensablecomponent, which is readily sorbed into the polymer matrix, swells or,in the case of a glassy polymer, plasticizes the membrane, therebyreducing its selective capabilities. A technique for predicting mixedgas performance under real conditions from pure gas measurements withany reliability has not yet been developed.

A good example of these performance problems is the separation ofhydrogen from mixtures containing hydrogen, methane and otherhydrocarbons. Increasing reliance on low-hydrogen, high-sulfur crudes,coupled with tighter environmental regulations, has raised hydrogendemand in refineries. This is primarily due to increasedhydrodesulfurization and hydrocracking; as a result many refineries arenow out of balance with respect to hydrogen supply. At the same time,large quantities of hydrogen-containing off-gas from refinery processesare currently rejected to the refinery's fuel gas systems. Besides beinga potential source of hydrogen, these off-gases contain hydrocarbons ofvalue, for example, as liquefied petroleum gas (LPG) and chemicalfeedstocks.

The principal technologies available to recover hydrogen from theseoff-gases are cryogenic separation, pressure swing adsorption (PSA), andmembrane separation. Membrane gas separation, the newest, is based onthe difference in permeation rates of gas components through a selectivemembrane. Many membrane materials are much more permeable to hydrogenthan to other gases and vapors. One of the first applications of gasseparation membranes was recovery of hydrogen from ammonia plant purgestreams, which contain hydrogen and nitrogen. This is an idealapplication for membrane technology, because the membrane selectivity ishigh, and the feed gas is clean (free of contaminants, such as heavierhydrocarbons). Another successful application is to adjusthydrogen/carbon monoxide or hydrogen/methane ratios for synthesis gasproduction. Again, the feed gas is free of heavy hydrocarbon compounds.

Application of membranes to refinery separation operations has been muchless successful. Refinery gas streams contain contaminants such as watervapor, acid gases, olefins, aromatics, and other hydrocarbons. Atrelatively low concentrations, these contaminants cause membraneplasticization and loss of selectivity. At higher concentrations theycan condense on the membrane and cause irreversible damage to it. When afeedstrearn containing such components and hydrogen is introduced into amembrane system, the hydrogen is removed from the feed gas into thepermeate and the gas remaining on the feed side becomes progressivelyenriched in hydrocarbons, raising the dewpoint. For example, if thetotal hydrocarbon content increases from 60% in the feed gas to 85% inthe residue gas, the dewpoint may increase by as much as 25° C. or more,depending on hydrocarbon mix. Maintaining this hydrocarbon-rich mixtureas gas may require it to be maintained at high temperature, such as 60°C., 70° C., 80° C. or even higher, which is costly and may itselfeventually adversely affect the mechanical integrity of the membrane.Failure to do this means the hydrocarbon stream may enter theliquid-phase region of the phase diagram before it leaves the membranemodule, and condense on the membrane surface, damaging it beyondrecovery. Even if the hydrocarbons are kept in the gas phase, separationperformance may fall away completely in the presence of hydrocarbon-richmixtures. These issues are discussed, for example, in J. M. S. Henis,“Commercial and Practical Aspects of Gas Separation Membranes” Chapter10 of D. R. Paul and Y. P. Yampol'skii, Polymeric Gas SeparationMembranes, CRC Press, Boca Raton, 1994. This reference gives upperlimits on various contaminants in streams to be treated by polysulfonemembranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation ofaromatics, 25% saturation of olefins and 11° C. above paraffin dewpoint(pages 473-474).

A great deal of research has been performed on improved membranematerials for hydrogen separation. A number of these materials appear tohave significantly better properties than the original cellulose acetateor polysulfone membranes. For example, modem polyimide membranes havebeen reported with selectivity for hydrogen over methane of 50 to 200,as in U.S. Pat. Nos. 4,880,442 and 5,141,642. Unfortunately,thesematerials appear to remain susceptible to severe loss of performancethrough plasticization and to catastrophic collapse if contacted byliquid hydrocarbons. Several failures have been reported in refineryapplications where these conditions occur. This low process reliabilityhas caused a number of process operators to discontinue applications ofmembrane separation for hydrogen recovery.

Another example of an application in which membranes have difficultydelivering and maintaining adequate performance is the removal of carbondioxide from natural gas. Natural gas provides more than one-fifth ofall the primary energy used in the United States, but much raw gas is“subquality”, that is, it exceeds the pipeline specifications innitrogen, carbon dioxide and/or hydrogen sulfide content. In particular,about 10% of gas contains excess carbon dioxide. Membrane technology isattractive for removing this carbon dioxide, because many membranematerials are very permeable to carbon dioxide, and because treatmentcan be accomplished using the high wellhead gas pressure as the drivingforce for the separation. However, carbon dioxide readily sorbs into andinteracts strongly with many polymers, and in the case of gas mixturessuch as carbon dioxide/methane with other components, the expectation isthat the carbon dioxide at least will have a swelling or plasticizingeffect, thereby adversely changing the membrane permeationcharacteristics. These issues are again discussed in the Henis referencecited above.

In the past, cellulose acetate, which can provide a carbondioxide/methane selectivity of about 10-20 in gas mixtures at pressure,has been the membrane material of choice for this application, and about100 plants using cellulose acetate membranes are believed to have beeninstalled. Nevertheless, cellulose acetate membranes are not withoutproblems. Natural gas often contains substantial amounts of water,either as entrained liquid, or in vapor form, which may lead tocondensation within the membrane modules. However, contact with liquidwater can cause the membrane selectivity to be lost completely, andexposure to water vapor at relative humidities greater than about 20-30%can cause irreversible membrane compaction and loss of flux. Thepresence of hydrogen sulfide in conjunction with water vapor is alsodamaging, as are high levels of C₃₊ hydrocarbons. These issues arediscussed in more detail in U.S. Pat. No. 5,407,466, columns 2-6, whichpatent is incorporated herein by reference.

Yet another challenging area is the separation of mixtures of lighthydrocarbon vapors. For example, olefins, particularly ethylene andpropylene, are important chemical feedstocks. About 17.5 million tons ofethylene and 10 million tons of propylene are produced in the UnitedStates annually, much as a by-product of petrochemical processing.Before they can be used, the raw olefins must be separated from mixturescontaining saturated hydrocarbons and other components. Currently,separation of olefin/paraffin mixtures is usually carried out bydistillation. The low relative volatilities of the components make thisprocess costly and complicated; distillation columns are typically up to300 feet tall and the process is very energy-intensive. More economicalseparation processes are needed. Using a membrane to separate olefinsfrom paraffins is an alternative to distillation that has beenconsidered. However, the separation is difficult because of the similarmolecular sizes and condensabilities of the components, as well as thechallenge of operating the membranes in a hydrocarbon-rich environment,and no material that can provide adequate performance with real vapormixtures under pressure has been found.

Thus, the need remains for membranes that will provide and maintainadequate performance under the conditions of exposure to hydrocarbons,and particularly C₃₊ hydrocarbons, that are commonplace in refineries,chemical plants, or gas fields.

Films or membranes made from fluorinated polymers having a ringstructure in the repeat unit are known. For example:

1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclosevarious perfluorinated polymers having repeating units of perfluorinatedcyclic ethers, and cite the gas-permeation properties of certain ofthese, as in column 8, lines 48-60 of 4,910,276.

2. A paper entitled “A study on perfluoropolymer purification and itsapplication to membrane formation”(V. Arcella et al., Journal ofMembrane Science, Vol. 163, pages 203-209 (1999)) discusses theproperties of membranes made from a copolymer of tetrafluoroethylene anda dioxole. Gas permeation data for various gases are cited.

3. European Patent Application 0 649 676 A1, to L'Air Liquide, disclosespost-treatment of gas separation membranes by applying a layer offluoropolymer, such as a perfluorinated dioxole, to seal holes or otherdefects in the membrane surface.

4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methodsusing perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patentalso discloses comparative data for membranes made fromperfluoro(2-methylene-4-methyl- 1,3-dioxolane) polymer (Example XI).

5. A paper entitled “Gas and vapor transport properties of amorphousperfluorinated copolymer membranes based on2,2-bistrifluoromethyl-4,5-difluoro- 1,3-dioxole/tetrafluoroethylene”(I.Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133(1996)) discusses the free volume and gas permeation properties offluorinated dioxole/tetrafluoroethylene copolymers compared withsubstituted acetylene polymers. This reference also shows thesusceptibility of this dioxole polymer to plasticization by organicvapors and the loss of selectivity as vapor partial pressure in a gasmixture increases (FIGS. 3 and 4).

Most of the data reported in the prior art references listed above arefor permanent gases, carbon dioxide and methane, and refer only tomeasurements made with pure gases. The data reported in item 5 indicatethat even these fluorinated polymers, which are characterized by theirchemical inertness, appear to be similar to conventionalhydrogen-separating membranes in their inability to withstand exposureto propane and heavier hydrocarbons.

SUMMARY OF THE INVENTION

The invention is a process for separating a gas from a gas mixturecontaining a C₃₊ hydrocarbon vapor. The separation is carried out byrunning a stream of the gas mixture across a membrane that is selectivefor the desired gas to be separated over the C₃₊ hydrocarbon vapor. Theprocess results, therefore, in a permeate stream enriched in the desiredgas and a residue stream depleted in that gas. The process differs fromprocesses previously available in the art in that:

(i) the membranes are able to maintain useful separation properties inthe presence of C₃₊ hydrocarbon vapor at high levels in the gas mixture,and

(ii) the membranes can recover from accidental exposure to liquidorganic compounds.

To quantify these attributes, the membranes used in the process of theinvention are characterized in terms of their selectivity before andafter exposure to liquid hydrocarbons. Specifically, the membranes havea post-exposure selectivity for the desired gas over the C₃₊ hydrocarbonvapor, after exposure of the separation membrane to a liquidhydrocarbon, for example, toluene, and subsequent drying, that is atleast about 60%, 65% or even 70% of a pre-exposure selectivity for thedesired gas over the C₃₊ hydrocarbon vapor, the pre- and post-exposureselectivities being measured with a test gas mixture of the samecomposition and under like conditions.

At least the selective layer responsible for the gas discriminatingproperties of the membranes is made from an amorphous glassy polymer orcopolymer. The polymer is fluorinated, generally heavily fluorinated, bywhich we mean having a fluorine:carbon ratio of atoms in the polymer ofat least about 1:1. Preferably, the polymer is perfluorinated.Typically, the polymer is further characterized by a fractional freevolume no greater than about 0.3 and by a glass transition temperature,Tg, of at least about 100° C.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising a desired gas and a C₃₊ hydrocarbonvapor across the feed side of a separation membrane having a feed sideand a permeate side, the separation membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor the desired gas over the C₃₊ hydrocarbon vapor, after exposure ofthe separation membrane to liquid toluene and subsequent drying, that isat least about 65% of a pre-exposure selectivity for the desired gasover the C₃₊ hydrocarbon vapor, as measured pre- and post-exposure witha test gas mixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched in thedesired gas compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in thedesired gas compared to the gas mixture.

The permeating desired gas may be either a valuable gas that it isdesired to retrieve as an enriched product, or a contaminant that it isdesired to remove. Thus either the permeate stream or the residuestream, or both, may be the useful products of the process. Gases thatmay be separated from C₃₊ hydrocarbons by the process include, but arenot limited to, hydrogen, nitrogen, oxygen, air, argon, carbon dioxide,methane, ethane, light olefins and light hydrocarbon isomers. Examplesof C₃₊ hydrocarbon vapors from which the gas may be separated include,but are not limited to paraffins, both straight and branched, forexample, propane, butanes, pentanes, hexanes; olefins and otheraliphatic unsaturated organics, for example, propylene, butene; aromatichydrocarbons, for example, benzene, toluene, xylenes; vapors ofhalogenated solvents, for example, methylene chloride,perchloroethylene; alcohols; ketones; and diverse other volatile organiccompounds.

Various materials may be used for the polymeric selective layer to meetthe characterizing requirements. These include, but are not limited to:

(i) polymers comprising a fluorinated dioxole monomer;

(ii) polymers comprising a fluorinated dioxolane monomer;

(iii) polymers comprising a fluorinated alkyl ether monomer;

(iv) perfluorinated polyimides;

(v) amorphous copolymers of tetrafluoroethylene.

Particularly preferred materials for the selective layer of the membraneused to carry out the process of the invention are copolymers of afluorinated dioxole with tetrafluoroethylene. A specific most preferredmaterial is such a copolymer having the structure:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.

Contrary to what would be expected from the data presented in the Pinnauet al. Journal of Membrane Science paper, we have unexpectedly foundthat membranes formed from such fluorinated dioxole/tetrafluoroethylenecopolymers can withstand exposure to C₃₊ hydrocarbons well enough toprovide useful separation capability for gas mixtures that include C₃₊hydrocarbons. This resistance persists even when the C₃₊ hydrocarbonsare present at high levels, such as 5%, 10%, 15% or even more.

A particularly important advantage of the invention is that themembranes can retain selectivity for desired gases, such as hydrogen,nitrogen, carbon dioxide, methane, or light olefin, even in the presenceof streams rich in, or even essentially saturated with, C₃₊hydrocarbons. This distinguishes these membrane materials from all othermembrane materials previously used commercially for such separations.Membranes made from fluorinated dioxoles have been believed previouslyto behave like conventional membrane materials in suffering fromdebilitating plasticization in a hydrocarbon containing environment, tothe point that they may even become selective for hydrocarbons overpermanent gas even at moderate C₃₊ hydrocarbon partial pressures. Wehave discovered that this is not the case for the membranes taughtherein. This unexpected result is achieved because the membranes used inthe invention are unusually resistant to plasticization by hydrocarbonvapors.

The membranes are also resistant to contact with liquid hydrocarbons, inthat they are able to retain their selectivity for hydrogen over methaneafter transient or even prolonged exposure to liquid aromatics, forexample. This is a second beneficial characteristic that differentiatesthe processes of the invention from prior art processes. In the past,exposure of the membranes to liquid hydrocarbons frequently meant thatthe membranes were irreversibly damaged and had to be removed andreplaced.

These unexpected and unusual attributes render the process of theinvention useful in situations where it was formerly difficult orimpractical for membrane separation to be used, or where membranelifetimes were poor.

Because the preferred polymers are glassy and rigid, an unsupported filmof the polymer may be usable in principle as a single-layer gasseparation membrane. However, such layer will normally be far too thickto yield acceptable transmembrane flux, and in practice, the separationmembrane usually comprises a very thin selective layer that forms partof a thicker structure, such as an asymmetric membrane or a compositemembrane. The making of these types of membranes is well known in theart. If the membrane is a composite membrane, the support layer mayoptionally be made from a fluorinated polymer also, making the membranea totally fluorinated structure and enhancing chemical resistance. Themembrane may take any form, such as hollow fiber, which may be potted incylindrical bundles, or flat sheets, which may be mounted inplate-and-frame modules or formed into spiral-wound modules.

The driving force for permeation across the membrane is the pressuredifference between the feed and permeate sides, which can be generatedin a variety of ways. The pressure difference may be provided bycompressing the feedstream, drawing a vacuum on the permeate side, or acombination of both. The membrane is able to tolerate high feedpressures, such as above 200 psia, 300 psia, 400 psia or more. Asmentioned above, the membrane is able to operate satisfactorily in thepresence of C₃₊ hydrocarbons at high levels. Thus, the partial pressuresof the hydrocarbons in the feed may be close to saturation. For example,depending on the mix of hydrocarbons and the temperature of the gas, theaggregate partial pressure of all C₃₊ hydrocarbons in the gas might beas much as 10 psia, 15 psia, 25 psia, 50 psia, 100 psia, 200 psia ormore. Expressed as a percentage of the saturation vapor pressure at thattemperature, the partial pressure of hydrocarbons, particularly C₃₊hydrocarbons, may be 20%, 30%, 50% or even 70% or more of saturation.

The membrane separation process may be configured in many possible ways,and may include a single membrane unit or an array of two or more unitsin series or cascade arrangements. The processes of the invention alsoinclude combinations of the membrane separation process defined abovewith other separation processes, such as adsorption, absorption,distillation, condensation or other types of membrane separation.

In another aspect, the invention is a process for separating hydrogenfrom hydrocarbons in a multicomponent mixture containing at leasthydrogen and one or more C₃₊ hydrocarbon vapors. Such a mixture mighttypically, but not necessarily, be found as a petrochemical plant or arefinery process or waste stream, such as streams from reformers,crackers, hydroprocessors and the like.

The process involves running a stream containing hydrogen across thefeed side of a membrane that is selectively permeable to the hydrogenover the hydrocarbons in the stream. The hydrogen is concentrated in thepermeate stream; the residue stream is thus correspondingly depleted ofhydrogen. The process can separate hydrogen from methane, hydrogen fromC₂₊ hydrocarbons, hydrogen from C₃₊ hydrocarbon vapors, or anycombination of these.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising hydrogen and a C₃₊ hydrocarbonvapor across the feed side of a separation membrane having a feed sideand a permeate side, the separation membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor hydrogen over the C₃₊ hydrocarbon vapor, after exposure of theseparation membrane to liquid toluene and subsequent drying, that is atleast about 65% of a pre-exposure selectivity for hydrogen over the C₃₊hydrocarbon vapor, as measured pre- and post-exposure with a test gasmixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched inhydrogen compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in hydrogencompared to the gas mixture.

The process differs from previous hydrogen/hydrocarbon separationprocesses in the nature of the membrane that is used. The membranes are,as described above, able to maintain useful separation properties in thepresence of organic vapors at high activity, and able to recover fromaccidental exposure to liquid hydrocarbons.

The scope of the invention in this aspect is not intended to be limitedto any particular gas streams, but to encompass any situation where agas stream containing hydrogen and hydrocarbon gas is to be separated.The composition of the gas may vary widely, from a mixture that containsminor amounts of hydrogen in admixture with various hydrocarboncomponents, including relatively heavy hydrocarbons, such as C₅-C₈hydrocarbons or heavier, to a mixture of mostly hydrogen, such as 80%hydrogen, 90% hydrogen or above, with methane and other very lightcomponents.

The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for hydrogen over methane of at least about 10, for hydrogen overpropane of at least about 50, and for hydrogen over n-butane of at leastabout 100. Frequently, the hydrogen/methane selectivity achieved is 20or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

In yet another aspect, the invention is a process for separating olefinsfrom paraffins, particularly propylene from propane. Such mixtures arefound as olefin manufacturing effluent streams, and in variouspetrochemical plant streams, for example.

The process involves running a stream comprising propylene and propaneacross the feed side of a membrane that is selectively permeable topropylene. The propylene is concentrated in the permeate stream; theresidue stream is thus correspondingly depleted of propylene.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising propylene and propane across thefeed side of a separation membrane having a feed side and a permeateside, the membrane having a selective layer comprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor propylene over propane, after exposure of the separation membrane toliquid toluene and subsequent drying, that is at least about 65% of apre-exposure selectivity for propylene over propane as measured pre- andpost-exposure with a test gas mixture of the same composition and underlike conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched inpropylene compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted inpropylene compared to the gas mixture.

The process typically provides a propylene/propane selectivity of atleast about 2.5, and more preferably at least about 3, which can besustained, even with streams composed entirely of C₃₊ hydrocarbons, overa range of pressures.

In yet another aspect, the invention is a process for separating carbondioxide from methane and other hydrocarbons. Such a mixture might beencountered during the processing of natural gas, of associated gas fromoil wells, or of certain petrochemical streams, for example.

The process involves running a stream containing carbon dioxide acrossthe feed side of a membrane that is selectively permeable to the carbondioxide over methane and the other hydrocarbons in the stream. Thecarbon dioxide is concentrated in the permeate stream; the residuestream is thus correspondingly depleted of carbon dioxide.

The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for carbon dioxide over methane of at least about 5, even at high carbondioxide activity. Frequently, the carbon dioxide/methane selectivityachieved is 10 or more, and may be as much as 15 or more, even in thepresence of significant concentrations of C₂₊ hydrocarbons.

Other separation processes that can be carried out within the scope ofthe invention include, but are not limited to, separation of otherpermanent gases, for example, nitrogen, oxygen, air or argon, fromorganics; separation of methane from C₃₊ organics; and separation ofisomers from one another.

It is an object of the present invention to provide a membrane-basedprocess for separation of gases from gas mixtures containing C₃₊hydrocarbon vapors. Additional objects and advantages of the inventionwill be apparent from the description below to those of ordinary skillin the art.

It is to be understood that the above summary and the following detaileddescription are intended to explain and illustrate the invention withoutrestricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the process of the invention inits most basic form.

FIG. 2 is a graph of pressure-normalized pure-gas flux of hydrogen,nitrogen and several light hydrocarbons as a function of pressure forcomposite membranes having Hyflon® AD60 selective layers.

FIG. 3 is a graph of calculated nitrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 2.

FIG. 4 is a graph of calculated hydrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 2.

FIG. 5 is a graph of pressure-normalized pure-gas flux of hydrogen,nitrogen and several light hydrocarbons as a function of pressure forcomposite membranes having Hyflon® AD80 selective layers.

FIG. 6 is a graph of calculated hydrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 5.

FIG. 7 is a graph of calculated nitrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 5.

FIG. 8 is a graph of pressure-normalized pure-gas flux of hydrogen,nitrogen and several light hydrocarbons as a function of pressure forcomposite membranes having Teflon® AF 2400 selective layers.

FIG. 9 is a graph of calculated nitrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 8.

FIG. 10 is a graph of calculated hydrogen/hydrocarbon selectivity basedon the pure gas data of FIG. 8.

FIG. 11 is a graph of pressure-normalized mixed-gas flux of carbondioxide, methane and propane as a function of pressure for compositemembranes having Hyflon® AD 60 selective layers.

FIG. 12 is a graph of mixed-gas carbon dioxide/methane and carbondioxide/propane selectivity based on the mixed gas data of FIG. 11.

FIG. 13 is a graph of pressure-normalized mixed-gas flux of hydrogen andseveral light hydrocarbons as a function of pressure for compositemembranes having Hyflon® AD 60 selective layers.

FIG. 14 is a graph of mixed-gas hydrogen/hydrocarbon selectivities basedon the mixed gas data of FIG. 13.

FIG. 15 is a graph of pressure-normalized mixed-gas flux of methane andn-butane as a function of n-butane concentration for composite membraneshaving Hyflon® AD 60 selective layers.

FIG. 16 is a graph of mixed-gas methane/n-butane selectivity based onthe mixed gas data of FIG. 15.

FIG. 17 is a graph of pressure-normalized mixed-gas flux of methane andn-butane as a function of n-butane concentration for composite membraneshaving Hyflon® AD 80 selective layers.

FIG. 18 is a graph of mixed-gas methane/n-butane selectivity based onthe mixed gas data of FIG. 17.

FIG. 19 is a graph of pressure-normalized mixed-gas flux of methane andn-butane as a function of n-butane concentration for composite membraneshaving Teflon® AF2400 selective layers.

FIG. 20 is a graph of mixed-gas methane/n-butane selectivity based onthe mixed gas data of FIG. 19.

FIG. 21 is a graph of mixed-gas carbon dioxide/methane selectivity as afunction of percent saturation of the gas mixture.

FIG. 22 is a graph of pressure-normalized mixed-gas flux of carbondioxide at 20° C. as a function of pressure for composite membraneshaving Hyflon® AD 60 selective layers.

FIG. 23 is a graph of mixed-gas carbon dioxide/methane selectivity basedon the mixed-gas data of FIG. 22.

FIG. 24 is a graph of pressure-normalized mixed-gas flux of carbondioxide at −20° C. as a function of pressure for composite membraneshaving Hyflon® AD 60 selective layers.

FIG. 25 is a graph of mixed-gas carbon dioxide/methane selectivity basedon the mixed-gas data of FIG. 24.

FIG. 26 is a graph of mixed-gas carbon dioxide/methane selectivity as afunction of percent saturation of the gas mixture, based on themixed-gas data of FIGS. 23 and 25.

FIG. 27 is a graph of pressure-normalized mixed-gas flux of propylene asa function of pressure for a spiral-wound module containing Hyflon® AD60 membranes.

FIG. 28 is a graph of mixed-gas propylene/propane selectivity based onthe mixed-gas data of FIG. 27.

FIG. 29 is a graph of pressure-normalized mixed-gas fluxes of propyleneand propane as a function of percent saturation of the gas mixture for aspiral-wound module containing Hyflon® AD 60 membranes.

FIG. 30 is a graph of pressure-normalized mixed-gas flux of propylene asa function of pressure for a spiral-wound module containing BPDA-TMPDpolyimide membranes.

FIG. 31 is a graph of pressure-normalized mixed-gas flux of propane as afunction of pressure for a spiral-wound module containing BPDA-TMPDpolyimide membranes.

FIG. 32 is a graph of pressure-normalized mixed-gas flux of nitrogen anddimethylethylamine as a function of dimethylethylamine concentration forcomposite membranes having Hyflon® AD 60 selective layers.

FIG. 33 is a graph of mixed-gas nitrogen/dimethylethylamine selectivitybased on the mixed-gas data of FIG. 32.

FIG. 34 is a graph of pressure-normalized mixed-gas flux of nitrogen andtriethylamine as a function of triethylamine concentration for compositemembranes having Hyflon® AD 60 selective layers.

FIG. 35 is a graph of mixed-gas nitrogen/triethylamine selectivity basedon the mixed-gas data of FIG. 34.

FIG. 36 is a schematic drawing of the process of the invention appliedto treatment of refinery off-gas.

FIG. 37 is a schematic drawing of the process of the invention appliedto treatment of an olefin/paraffin mixture from a petrochemicalmanufacturing plant.

DETAILED DESCRIPTION OF THE INVENTION

The term gas as used herein means a gas or a vapor.

The terms hydrocarbon and organic vapor or organic compound are usedinterchangeably herein, and include, but are not limited to, saturatedand unsaturated compounds of hydrogen and carbon atoms in straightchain, branched chain and cyclic configurations, including aromaticconfigurations, as well as compounds containing oxygen, nitrogen,halogen or other atoms.

The term C₂₊ hydrocarbon means a hydrocarbon having at least two carbonatoms; the term C₃₊ hydrocarbon means a hydrocarbon having at leastthree carbon atoms; and so on.

The term light hydrocarbon means a hydrocarbon molecule having no morethan about six carbon atoms.

The term heavier hydrocarbon means a C₃₊ hydrocarbon.

All percentages herein are by volume unless otherwise stated.

The invention is a process for separating a gas from a gas mixturecontaining a C₃₊ hydrocarbon vapor. The separation is carried out byrunning a stream of the gas mixture across a membrane that is selectivefor the desired gas to be separated over the C₃₊ hydrocarbon vapor. Theprocess results, therefore, in a permeate stream enriched in the desiredgas and a residue stream depleted in that gas.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising a desired gas and a C₃₊ hydrocarbonvapor across the feed side of a separation membrane having a feed sideand a permeate side, the separation membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor the desired gas over the C₃₊ hydrocarbon vapor, after exposure ofthe separation membrane to liquid toluene and subsequent drying, that isat least about 65% of a pre-exposure selectivity for the desired gasover the C₃₊ hydrocarbon vapor, as measured pre- and post-exposure witha test gas mixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched in thedesired gas compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in thedesired gas compared to the gas mixture.

The feed gas mixture to be separated often contains additionalcomponents, such as methane or ethylene, as well as C₃₊ hydrocarbons,that will be separated from the desired gas by the process. In thiscase, the process results in a permeate stream enriched in the desiredgas and depleted in both the C₃₊ hydrocarbon and the additionalcomponent compared with the feed gas mixture, and a residue streamdepleted in the desired gas and enriched in both the C₃₊ hydrocarbon andthe additional component compared with the feed gas mixture.

The process differs from processes previously available in the art inthat:

(i) the membranes are able to maintain useful separation properties inthe presence of organic vapors, such as C₃₊ hydrocarbons, even at highlevels in the gas, and

(ii) the membranes can recover from accidental exposure to liquidorganic compounds.

To quantify these attributes, the membranes used in the process of theinvention are characterized in terms of their selectivity before andafter exposure to liquid hydrocarbons. Specifically, the membranes havea post-exposure selectivity for the desired gas over the C₃₊ hydrocarbonvapor, after exposure of the separation membrane to a liquidhydrocarbon, for example, toluene, and subsequent drying, that is atleast about 60%, 65% or even 70% of a pre-exposure selectivity for thedesired gas over the C₃₊ hydrocarbon vapor, the pre- and post-exposureselectivities being measured with a test gas mixture of the samecomposition and under like conditions.

In applying this test to determine whether the membrane is suitable foruse in the process of the invention, it is important to test themembrane itself, and not just a film of the selective layer polymer. Athick film, for example 50 μm or more thick, of the selective layerpolymer may appear to resist dissolution and swelling and maintainselectivity, even when soaked for days in liquid hydrocarbon. However,when used in an asymmetric or composite membrane with a selective layerthin enough to provide useful transmembrane flux for the desired gas(which may mean a selective layer as thin as 10 μm, 5 μm, 1 μm or less),the same material may disintegrate within minutes of contact with thehydrocarbon liquid.

It is also important that the test gas mixtures used to measure theselectivity before and after exposure have essentially the samecomposition, and that the test be carried out under essentially the sameconditions of pressure, temperature, gas flow and membrane area, sinceall of these parameters may have an effect on selectivity. The test gasmixture should obviously contain the desired gas and the C₃₊ hydrocarbonvapor, for example, propane, propylene or benzene, from which thedesired gas is to be separated in the process, but need not be identicalin composition to the feed gas mixture to the process, since this maychange from time to time in any case.

It is preferred that the hydrocarbon liquid to which the membrane isexposed in the test is an aromatic liquid, such as toluene, rather thana paraffin, for example, since this provides more aggressive testconditions. The test can be carried out in any convenient manner. Asimple and preferred protocol is to measure the membrane selectivityusing a bench-top test cell apparatus such as is familiar to those ofskill in the art, remove the membrane stamp from the test cell,immersing it in liquid toluene for a period, remove it, dry it in airand retest it as before. For an adequate test, the period of immersionshould be representative of the exposure that might occur during asystem upset when the membrane is in use, such as one or two hours, orovernight (about eight hours).

At least the selective layer responsible for the gas discriminatingproperties of the membrane is made from a glassy polymer or copolymer.The polymer should be substantially amorphous. Crystalline polymers aretypically essentially insoluble and thus render membrane makingdifficult, as well as exhibiting generally very low gas permeabilities.Crystalline polymers are not normally suitable for the selective layer,therefore.

The selective layer polymer should be fluorinated, and generally thedegree of fluorination should be high, to increase the chemicalinertness and resistance of the material. By high, we mean having afluorine:carbon ratio of atoms in the polymer of at least 1:1. Mostpreferably, the polymer is perfluorinated, even if the perfluorinatedstructure has a less than 1:1 fluorine:carbon ratio.

Since the polymers used for the selective layer need to remain rigid andglassy during operation, they should also have glass transitiontemperatures comfortably above temperatures to which they are typicallyexposed during the process. Polymers with glass transition temperatureabove about 100° C. are preferred, and, subject also to the otherrequirements and preferences above, the higher the glass transitiontemperature, in other words, the more rigid the polymer, the morepreferred it is.

Polymers that have been used in commercial gas-separation systems todate are not suitable for use as selective layer materials insofar asthey are unable to meet the characterizing requirements of resistance toC₃₊ hydrocarbons. Such polymers include cellulose acetate, polysulfoneand polyimides that are not perfluorinated. As discussed above and shownin the experimental examples below, these materials lose their flux, orselectivity, or both, properties as a result of exposure to C₃₊hydrocarbons at high activity or prolonged exposure to hydrocarbons atlower activity.

Various materials may be used for the polymeric selective layer to meetthe characterizing requirements. These include, but are not limited to:

(i) polymers comprising a fluorinated dioxole monomer;

(ii) polymers comprising a fluorinated dioxolane monomer;

(iii) polymers comprising a fluorinated alkyl ether monomer;

(iv) perfluorinated polyimides; and

(v) polymers incorporating tetrafluoroethylene units.

None of these are new polymers in themselves. In fact, general polymerformulations embracing those suitable for use in the invention aredescribed in patents dating back from the present day to the 1960s, forexample, U.S. Pat. Nos. 3,308,107; 3,488,335; 3,865,845; 4,399,264;4,431,786; 4,565,855; 4,594,399; 4,754,009; 4,897,457; 4,910,276;5,021,602; 5,117,272; 5,268,411; 5,498,682; 5,510,406; 5,710,345;5,883,177; 5,962,612; and 6,040,419.

Not all individual polymers within the above structural definitions andpreferences are suitable for use as membrane selective layers in theinvention. For example, homopolymers of tetrafluoroethylene are verychemically inert and, thus, resistant to plasticization. However, attemperatures of interest for membrane gas separations, they tend to becrystalline or semi-crystalline. As a result, the gas permeabilitiesthrough the polymer in non-porous form are too low to be of interest foruse for the selective layer of a gas-separation membrane. Incorporatedinto copolymers, however, they enhance chemical resistance and physicalrigidity. Therefore, combinations of tetrafluoroethylene with othermonomer units that result in overall amorphous, yet rigid, highlyfluorinated, copolymers are particularly preferred.

As a further example that some members of the polymer groups cited aboveare not suitable for practice of the invention, certain of the dioxolepolymers and copolymers of perfluoro-2,2-dimethyl-1,3-dioxole reportedin U.S. Pat. No. 5,051,114 have been shown to be susceptible toplasticization to the point of switching from being selective fornitrogen over hydrocarbons to being selective for hydrocarbons overnitrogen as the hydrocarbon partial pressure increases. These polymersare, however, characterized by very high fractional free volume withinthe polymer, typically above 0.3. For example, a paper by A. Yu.Alentiev et al, “High transport parameters and free volume ofperfluorodioxole copolymers”, Journal of Membrane Science, Vol. 126,pages 123-132 (1997) reports fractional free volumes of 0.32 and 0.37for two grades of perfluoro-2,2-dimethyl1,3-dioxole copolymers (Table 1,page 125). Likewise, these polymers are of low density compared withother polymers, such as below about 1.8 g/cm³ and are unusually gaspermeable, for instance, exhibiting pure gas permeabilities as high as1,000 Barrer or more for oxygen and as high as 2,000 Barrer or more forhydrogen. It is believed that polymers with denser chain packing, andthus lower fractional free volume, higher density and lowerpermeability, are more resistant to plasticization. Hence, the polymersused in the invention to form the selective, discriminating layer of themembrane should preferably be limited, in addition to the specificstructural limitations defined and discussed above, to those having afractional free volume less than about 0.3.

In referring to fractional free volume (FFV), we mean the free volumeper unit volume of the polymer, defined and calculated as:

FFV=SFV/ν_(sp)

where SFV is the specific free volume, calculated as:

SFV=ν_(sp)−ν₀=ν_(sp)−1.3ν_(W)

and where:

ν_(sp) is the specific volume (cm³/g) of the polymer determined fromdensity or thermal expansion measurements,

ν₀ is the zero point volume at 0° K, and

ν_(W) is the van der Waals volume calculated using the groupcontribution method of Bondi, as described in D. W. van Krevelan,Properties of Polymers, 3_(rd) Edition, Elsevier, Amsterdam, 1990, pages71-76.

Expressed in terms of density, the selective layer polymers shouldpreferably have a density above about 1.8 g/cm³. Expressed in terms ofpermeability, the selective layer polymers will generally exhibit anoxygen permeability no higher than about 300 Barrer, more typically nohigher than about 100 Barrer, and a hydrogen permeability no higher thanabout 1,000 Barrer, more typically no higher than about 500 Barrer.

Preferred polymers for the selective layer of the membrane are formedfrom fluorinated monomers of dioxoles, which are five-member rings ofthe form

where R₁, R₂, R₃, and R₄ each independently may be hydrogen, fluorine,other halogen, alkyl or other group, subject to the resulting polymermeeting the specified definition in terms of fractional free volume anddegree of fluorination. These materials polymerize by opening of thedouble bond and may take the form of homopolymers or copolymers withother repeat units. Copolymers that include tetrafluoroethylene unitsare particularly preferred.

Specific most preferred materials are copolymers of tetrafluoroethylenewith 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole having thestructure:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.

Such materials are available commercially from Ausimont S.p.A., ofMilan, Italy under the trade name Hyflon® AD. Different grades areavailable varying in proportions of the dioxole and tetrafluoroethyleneunits, with fluorine:carbon ratios of between 1.5 and 2, depending onthe mix of repeat units. For example, grade Hyflon AD 60 contains a60:40 ratio of dioxole to tetrafluoroethylene units, has a fractionalfree volume of 0.23, a density of 1.93 g/cm³ and a glass transitiontemperature of 121° C., and grade Hyflon AD 80 contains an 80:20 ratioof dioxole to tetrafluoroethylene units, has a fractional free volume of0.23, a density of 1.92 g/cm³ and a glass transition temperature of 134°C.

Similar types of preferred materials are dioxolanes, also five-memberrings but with the polymerizable bonds outside the main ring.

Alternative preferred materials are polymers prepared from highlyfluorinated alkyl ether monomers, especially those polymerizable intocyclic ether repeat units with five or six members in the ring. Amongthese, particularly preferred is the set of polyperfluoro (alkenyl vinylethers) including polyperfluoro (allyl vinyl ether) and polyperfluoro(butenyl vinyl ether). A specific most preferred material of this typehas the structure:

where n is a positive integer.

This material is available commercially from Asahi Glass Company, ofTokyo, Japan under the trade name Cytop® Cytop has a fractional freevolume of 0.21, a density of 2.03 g/cm³ and a glass transitiontemperature of 108° C., and a fluorine:carbon ratio of 1.7.

Yet another group of materials that is believed to contain usefulselective layer materials is perfluorinated polyimides. Such materialshave been investigated for use as optical waveguides, and theirpreparation is described, for example, in S. Ando et al.,“Perfluorinated polymers for optical waveguides”, CHEMTECH, December,1994. To be usable as membrane materials, the polyimides have to becapable of being formed into continuous films. Thus, polyimides thatincorporate ether or other linkages that give some flexibility to themolecular structure are preferred. Particular examples are polymerscomprising repeat units prepared from the perfluorinated dianhydride1,4-bis(3,4-dicarboxytrifluorophenoxy) tetrafluorobenzene (10FEDA) ,which has the structure:

Diamines with which 10FEDA can be reacted to form polyamic acids andhence polyimides include 4FMPD, which has the structure:

The resulting 10FEDA/4FMPD polyimide has the repeat unit structure:

The polymer chosen for the selective layer can be used to form films ormembranes by any convenient technique known in the art, and may takediverse forms. Because the polymers are glassy and rigid, an unsupportedfilm, tube or fiber of the polymer may be usable in principle as asingle-layer membrane. However, such single-layer films will normally betoo thick to yield acceptable transmembrane flux, and in practice, theseparation membrane usually comprises a very thin selective layer thatforms part of a thicker structure. This may be, for example, an integralasymmetric membrane, comprising a dense skin region that forms theselective layer and a microporous support region. Such membranes wereoriginally developed by Loeb and Sourirajan, and their preparation inflat sheet or hollow fiber form is now conventional in the art and isdescribed, for example, in U.S. Pat. Nos. 3,133,132 to Loeb, and4,230,463 to Henis and Tripodi.

As a further, and a preferred, alternative, the membrane may be acomposite membrane, that is, a membrane having multiple layers. Moderncomposite membranes typically comprise a highly permeable but relativelynon-selective support membrane, which provides mechanical strength,coated with a thin selective layer of another material that is primarilyresponsible for the separation properties. Typically, but notnecessarily, such a composite membrane is made by solution-casting thesupport membrane, then solution-coating the selective layer. Generalpreparation techniques for making composite membranes of this type arewell known, and are described, for example, in U.S. Pat. No. 4,243,701to Riley et al., incorporated herein by reference. If the membrane ismade in the form of a composite membrane, it is particularly preferredto use a fluorinated or perfluorinated polymer, such as polyvinylidenefluoride, to make the microporous support membrane. Again, the membranemay take flat-sheet, tube or hollow-fiber form. The most preferredsupport membranes are those with an asymmetric structure, which providesa smooth, comparatively dense surface on which to coat the selectivelayer. Support membranes are themselves frequently cast onto a backingweb of paper or fabric. As an alternative to coating onto a supportmembrane, it is also possible to make a composite membrane bysolution-casting the polymer directly onto a non-removable backing web,as mentioned above. In hollow-fiber form, multilayer composite membranesmay be made by a coating procedure as taught, for example, in U.S. Pat.Nos. 4,863,761; 5,242,636; and 5,156,888, or by using a double-capillaryspinneret of the type taught in U.S. Pat. Nos. 5,141,642 and 5,318,417.

The membrane may also include additional layers, such as a gutter layerbetween the microporous support membrane and the selective layer, or asealing layer on top of the selective layer. A gutter layer generallyhas two purposes. The first is to coat the support with a material thatseals small defects in the support surface, and itself provides asmooth, essentially defect-free surface onto which the selective layermay be coated. The second is to provide a layer of highly permeablematerial that can channel permeating molecules to the relatively widelyspaced pores in the support layer. Preferred materials for the gutterlayer are fluorinated or perfluorinated, to maintain high chemicalresistance through the membrane structure, and of very highpermeability. Particularly preferred for the gutter layer, although theyare unsuitable for the selective layer, are the perfluorinated dioxolepolymers and copolymers of U.S. Pat. No. 5,051,114 referred to above,having fractional free volume greater than 0.3 and extraordinarily highpermeability, such as copolymers of perfluoro-2,2-dimethyl-1,3-dioxoleand tetrafluoroethylene, available commercially as Teflon® AF fromDuPont Fluoroproducts of Wilmington, Delaware. Such materials, or anyothers of good chemical resistance that provide protection for theselective layer without contributing significant resistance to gastransport, are also suitable as sealing layers.

Multiple selective layers may also be used.

The thickness of the selective layer or skin of the membranes can bechosen according to the proposed use, but will generally be no thickerthan 10 μm, and typically no thicker than 5 μm. It is preferred that theselective layer be sufficiently thin that the membrane provide apressure-normalized hydrogen flux, as measured with pure hydrogen gas at25° C., of at least about 100 GPU (where 1 GPU=1×10⁻⁶ cm³(STP)/cm².s.cmHg), more preferably at least about 200 GPU and most preferably atleast about 400 GPU. In general, the membranes of the invention providetransmembrane gas fluxes that are high compared with membranes usingconventional hydrogen-separating materials, such as polyimides,cellulose acetate and polysulfone.

Once formed, the membranes exhibit a combination of good mechanicalproperties, thermal stability, and high chemical resistance. Thefluorocarbon polymers that form the selective layer are typicallyinsoluble except in perfluorinated solvents and are resistant to acids,alkalis, oils, low-molecular-weight esters, ethers and ketones,aliphatic and aromatic hydrocarbons, and oxidizing agents, making themsuitable for use not only in the presence of C₃₊ hydrocarbons, but inmany other hostile environments.

The membranes of the invention may be prepared in any known membraneform and housed in any convenient type of housing and separation unit.We prefer to prepare the membranes in flat-sheet form and to house themin spiral-wound modules. However, flat-sheet membranes may also bemounted in plate-and-frame modules or in any other way. If the membranesare prepared in the form of hollow fibers or tubes, they may be pottedin cylindrical housings or otherwise.

The membrane separation unit comprises one or more membrane modules. Thenumber of membrane modules required will vary according to the volume ofgas to be treated, the composition of the feed gas, the desiredcompositions of the permeate and residue streams, the operating pressureof the system, and the available membrane area per module. Systems maycontain as few as one membrane module or as many as several hundred ormore. The modules may be housed individually in pressure vessels ormultiple elements may be mounted together in a sealed housing ofappropriate diameter and length.

The process of the invention in its most basic form is shown in FIG. 1.Referring to this figure, a feedstream, 1, containing a gas mixtureincluding a desired gas and one or more C₃₊ hydrocarbon vapors, ispassed into membrane separation unit 2 and flows across the feed side ofmembrane 3, which is characterized by having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and byits resistance to C₃₊ hydrocarbons in vapor or liquid form.

Under a pressure difference between the feed and permeate sides of themembrane, the desired gas passes preferentially to the permeate side,and gas-enriched stream, 5, is withdrawn from the permeate side. Theremaining gas-depleted, organic-enriched residue stream, 4, is withdrawnfrom the feed side.

The composition and pressure at which the feedstream is supplied to themembrane modules varies depending on the source of the stream. If thefeed gas stream to be treated is at high pressure compared withatmospheric, such as 200 psia, 400 psia, 500 psia or above, theseparation may be effected simply by making use of this high pressure toprovide an adequate driving force and feed:permeate pressure ratio.Otherwise, a pressure difference can be provided by compressing the feedstream, by drawing a vacuum on the permeate side of the membrane, or acombination of both. Polymer membranes can typically withstand pressuredifferences between the feed and permeate side up to about 1,500-2000psi, so it might occasionally be necessary to let down the gas pressurebefore it can be fed to the membrane system.

An important consideration is the effect of hydrocarbons, particularlyC₃₊ hydrocarbons, in the feed stream. Unlike prior art membranes, themembranes of the invention can maintain useful gas/hydrocarbonseparation performance, in terms of transmembrane gas flux andselectivity, when exposed to high concentrations of such organics, evenwhen the gas mixture is close to saturation with these compounds. Thisis true with respect to a broad range of hydrocarbons, includingparaffins, olefins, aromatics, such as benzene, toluene and xylenes(BTEX), alcohols and chlorinated compounds. These properties aredifferent from those reported in the literature for dioxole membranes,as well as obtained with prior art conventional commercial membranematerials, such as cellulose acetate, polysulfone, or non-perfluorinatedpolyimide.

Even if condensation of organic liquid does accidentally occur from timeto time, the membrane unit can generally be purged with, for example, aninert gas such as nitrogen, and the membranes will frequently continuethereafter to exhibit adequate gas/hydrocarbon selectivity properties.

In contrast, prior art membranes in commercial use are generallyplasticized and irreversibly damaged by exposure to C₃₊ hydrocarbonvapors at any significant concentration, such as more than about 10%,20% or 25%, or at more modest concentrations, such as less than 10%, forprolonged periods, and cannot withstand even fleeting exposure tocondensed organic liquids.

As a rough general guide, expressed as a concentration, the feed gastreated by the process of the invention may have a hydrocarbons content,including C₃₊ hydrocarbons, of at least about 5%, 10%, 15%, 20% orhigher. Expressed in terms of partial pressure, the feed stream mayoften be acceptable with a partial pressure of C₃₊ hydrocarbons of ashigh as 15 psia, 25 psia, 50 psia, 100 psia or more, assuming a gastemperature of ambient or above; and the residue stream partial pressureof C₃₊ hydrocarbons together can often be as high as 50 psia, 100 psia,150 psia or 200 psia, again assuming a temperature of ambient or above.Expressed as the ratio of the feed pressure, P, to the saturation vaporpressure, P_(sat) of the gas mixture, which is an approximate measure ofthe activity of the gas, the feed gas may be supplied to the membraneseparation step at pressure and temperature conditions that result inthe percentage P/P_(sat) being at least about 25%, 30%, 50%, 60 %, 70%or higher. Methane and C₂ components, which tend to have low boilingpoints, and to be less condensable and less harmful in terms of theirplasticizing ability, can generally be present in any concentration.

Depending on the performance characteristics of the membrane, and theoperating parameters of the system, the process can be designed forvarying levels of gas purification and recovery. Single-stagegas-separation processes typically remove up to about 80-95% of thepreferentially permeating component from the feed stream and produce apermeate stream significantly more concentrated in that component thanthe feed gas. This degree of separation is adequate for manyapplications. If the residue stream requires further purification, itmay be passed to a second bank of modules for a second processing step.If the permeate stream requires further concentration, it may be passedto a second bank of modules for a second-stage treatment. Suchmultistage or multistep processes, and variants thereof, will befamiliar to those of skill in the art, who will appreciate that theprocess may be configured in many possible ways, including single-stage,multistage, multistep, or more complicated arrays of two or more unitsin series or cascade arrangements.

In light of their unusual and advantageous properties, the membranes andprocesses of the invention are useful for many separation applications.Specific examples include, but are not limited to separation ofpermanent gases, for example, nitrogen, oxygen, air, argon or hydrogen,from organics; separation of methane from C₃₊ organics; separation ofcarbon dioxide from organics; separation of light olefins from otherorganics; and separation of isomers from one another, such as n-butanefrom iso-butane.

Of particular importance, the membranes and processes of the inventionare useful for many applications where hydrogen is to be separated frommixtures containing hydrogen and one or more hydrocarbons. In anotheraspect, therefore, the invention is a process for treating refinery orpetrochemical plant streams containing hydrogen and hydrocarbons, toseparate hydrogen from the hydrocarbons.

The following list of applications of the invention in this aspect isexemplary, but not limiting: separation of hydrogen from methane andother light hydrocarbons in process and off-gas streams from:hydrocrackers; hydrotreaters of various kinds, includinghydrodesulfurization units; coking reactors; catalytic reformers;catalytic crackers; specific isomerization, alkylation and dealkylationunits; steam reformers; hydrogenation and dehydrogenation processes; andsteam crackers for olefin production, as well as in streams frommanufacture of primary petrochemicals, chemical intermediates, fuels,polymers, agricultural chemicals and the like.

The treatment process of this invention, with respect to FIG. 1,involves running a refinery, chemical plant or the like stream, 1,containing a hydrogen/hydrocarbon mixture, typically including hydrogen,methane and C₃₊ hydrocarbon vapors, across the feed side of a membraneseparation unit 2, containing a membrane characterized as before, 3,that is selectively permeable to the hydrogen over the methane and otherhydrocarbons in the stream. The hydrogen is concentrated in the permeatestream, 5; the residue stream, 4, is thus correspondingly depleted ofhydrogen.

In other words, the process of the invention includes the followingsteps:

(a) passing a gas mixture comprising hydrogen and a C₃₊ hydrocarbonvapor across the feed side of a separation membrane having a feed sideand a permeate side, the separation membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor hydrogen over the C₃₊ hydrocarbon vapor, after exposure of theseparation membrane to liquid toluene and subsequent drying, that is atleast about 65% of a pre-exposure selectivity for hydrogen over the C₃₊hydrocarbon vapor, as measured pre- and post-exposure with a test gasmixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched inhydrogen compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in hydrogencompared to the gas mixture.

The process differs from previous hydrogen/hydrocarbon separationprocesses in the nature of the membrane that is used. The membranes are,as described above, able to maintain useful separation properties in thepresence of organic vapors, particularly C₃₊ hydrocarbons, at highpartial pressure, and able to recover from accidental exposure to liquidhydrocarbons.

The scope of the invention in this aspect is not intended to be limitedto any particular gas streams, but to encompass any situation where agas stream containing hydrogen and hydrocarbon gas is to be separated.The composition of the gas may vary widely, from a mixture that containsminor amounts of hydrogen in admixture with various hydrocarboncomponents, including relatively heavy hydrocarbons, such as C₅-C₈hydrocarbons or heavier, to a mixture of mostly hydrogen, such as 80%hydrogen, 90% hydrogen or above, with methane and other very lightcomponents. Typical examples of compositions and pressures of feed gasessuitable for treatment by the process of the invention, include, but arenot limited to, mixtures of hydrogen with methane and C₂-C₈ paraffinsand olefins having a C₃₊ hydrocarbon vapor content of as much as 15-20%or more at a total feed pressure of 400 psia; mixtures of hydrogen andmethane of any composition and pressure; and mixtures of hydrogen withC₁-C₄ paraffins having a total hydrocarbon content of as much as 60% ormore at a total feed pressure of 500 psia.

The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for hydrogen over methane of at least about 10, for hydrogen overpropane of at least about 50, and for hydrogen over n-butane of at leastabout 100. Frequently, the hydrogen/methane selectivity achieved is 20or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

Applications range from those treating very large streams, such asseparation of hydrogen/light hydrocarbon mixtures in ethylene plant coldtrains, to those handling much smaller streams, including recovery ofhydrogen from vent streams generated by hydrogen reduction processes.

An exemplary use of the process is to recover hydrogen from refinerywaste streams containing less than about 40% hydrogen, and rich in C₃₊hydrocarbons, for which PSA or cryogenic condensation is noteconomically attractive. Prior to the availability of the presentprocess, the only use for such streams was generally as fuel. Theprocess of the invention can be used to recover a hydrogen-rich permeatestream, containing, for example, 90% hydrogen, from such a feed stream.A stream of such composition may be recompressed and used in otherrefinery unit operations, or subjected to further treatment to yieldhigh purity hydrogen as required. The hydrocarbon-rich residue streammay be piped to the fuel header, thereby reducing the volume of fuel gasproduced, or sent for LPG recovery, for example.

A second exemplary application is hydrogen and olefin recovery fromfluid catalytic cracking (FCC) off-gas. Such streams typically contain10-20% hydrogen at 100-250 psig, and are a potential source of lightolefins also. The process of the invention can be used to recover bothhydrogen and hydrocarbons from these streams. The hydrogen product,typically containing 80-90% hydrogen, can be used as is or sent foradditional treatment. The hydrocarbon-rich residue can be used as fuel,or can be sent for olefin recovery from the hydrocarbon mixture bycryogenic distillation or the like.

Other exemplary applications include separation of hydrogen from steamcracker product gas generated by the manufacture of ethylene orpropylene by cracking ethane or propane with steam, and selectivepurging of reactor loops to remove methane or other contaminants fromthe loop. Such selective purging processes are discussed with respect toother types of membranes in U.S. Pat. No. 6,171,472, for example.

The invention has been described in this aspect as it relates to theseparation of hydrogen from hydrocarbon-containing gas mixtures.Processes that concern the separation of other permanent gases from gasmixtures containing hydrocarbons are also possible using the membranesas characterized above. Specific examples include, but are not limitedto, separation of nitrogen, oxygen or air from methane, ethylene andother organics; and separation of argon from ethylene.

For instance, as it relates to separation of nitrogen and/or oxygen fromorganic compounds, the invention includes the following steps:

(a) passing a gas mixture comprising nitrogen and/or oxygen, and a C₃₊hydrocarbon vapor across the feed side of a separation membrane having afeed side and a permeate side, the membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor nitrogen or oxygen over the C₃₊ hydrocarbon vapor, after exposure ofthe separation membrane to liquid toluene and subsequent drying, that isat least about 65% of a pre-exposure selectivity for nitrogen or oxygenover the C₃₊ hydrocarbon vapor, as measured pre- and post-exposure witha test gas mixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched innitrogen and/or oxygen compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in nitrogenand/or oxygen compared to the gas mixture.

Such a process is useful, for example, in separating nitrogen frommethane to treat natural gas that is out of specification by containingexcess nitrogen. In this case, the natural gas stream to be treatedcontains methane in addition to C₃₊ hydrocarbons. The process is alsouseful for treating gas mixtures in which nitrogen is to be separatedfrom ethylene, such as occur in polyolefin manufacturing. Yet anotheruse is to treat off-gases from numerous industrial processes thatproduce waste streams containing organic vapors in air or nitrogen, suchas arise when organic solvents are used in coating, spraying, cleaning,painting, or printing applications of all kinds, from organic liquidstorage tank vents, from chemical manufacturing, or from foundry coldboxes using organic catalysts for metal casting. In this case, diverseorganic vapors may be present in the stream, for example, halogenatedsolvents, alkyl amines, ketones or alcohols.

The process of the invention can provide a selectivity, in gas mixtures,for nitrogen over methane of as high as 2, 2.5 or even 3. Although thesenumbers seem small, they are remarkable, in that few prior art membranematerials offer any selectivity at all for nitrogen over methane. Forexample, polysulfone, cellulose acetate and polycarbonate all havenitrogen/methane selectivity of only about 1 or below, that is, theyoften exhibit slight methane/nitrogen selectivity. Polyimides, the bestgroup of prior art materials in this regard, offer typicalnitrogen/methane selectivity, even as measured with pure gases, only inthe range between 1 and 2.3.

The process of the invention can also provide exceptional selectivity,in gas mixtures, for nitrogen over ethylene of as high as 4, 5 or above.This performance is again unusual compared with other materials.

As it relates to separation of nitrogen from more complex organicmolecules, the process of the invention can provide much highermixed-gas selectivities, such as 20, 40, 50, 100 or higher, depending onthe nature of the organic compound and the process conditions.

Yet another possible permanent gas/organic compound separation is thatof argon from ethylene. A large number of chemical products are producedby catalytic oxidation of an appropriate organic feedstock. For example,ethylene oxide is made by oxidation of ethylene, as are acetaldehyde,vinyl acetate and vinyl chloride; propylene oxide and acrylonitrile areproduced by oxidation of propylene; benzoic acid by oxidation oftoluene; and caprolactam by oxidation of cyclohexane. Such oxidationprocesses operate in a loop, with modest conversion per pass, so thatlarge amounts of unreacted organic feedstock are recirculated back tothe reaction zone at each pass. The processes often use a feed ofoxygen-enriched air or high-purity oxygen as the oxygen source, leadingto a build-up of unreacted argon, which enters with the feed oxygen, inthe reactor loop. The process of the invention can be used toselectively purge argon from the loop, while retaining the ethylene,propylene or other organic feedstock for recycle to the process. In thisrespect, the process of the invention typically provides a selectivity,in gas mixtures, for argon over ethylene of as high as 4, 5, 6, 7 orabove. These are again very unusual and advantageous properties.

In yet another aspect, the invention is a process for separating carbondioxide from methane and C₃₊ hydrocarbon vapors. Such a mixture might beencountered during the processing of natural gas, of associated gas fromoil wells, or of certain petrochemical streams, for example.

For natural gas to be accepted into the pipeline, it must normallycontain no more than 4% carbon dioxide. As mentioned above, much rawnatural gas is out of specification in this regard, as well as being toorich in C₃₊ hydrocarbons content. Carbon dioxide rich streams also ariseas a result of oil extraction by miscible flood operations, in whichcarbon dioxide is injected into the formation to lower the viscosity ofthe oil. The resulting gas extracted with the oil is initially rich inmethane, but over time may contain as much as 80% or more carbondioxide.

The process of the invention may be used to treat such streams as shownagain with reference to FIG. 1. A stream containing carbon dioxide, 1,is passed across the feed side of a membrane separation unit, 2,containing a membrane as defined above, 3, that is selectively permeableto the carbon dioxide over methane and other hydrocarbons in the stream.The carbon dioxide is concentrated in the permeate stream, 5; theresidue stream, 4, is thus correspondingly depleted of carbon dioxideand enriched in hydrocarbons.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising carbon dioxide, methane and a C₃₊hydrocarbon vapor across the feed side of a separation membrane having afeed side and a permeate side, the membrane having a selective layercomprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor carbon dioxide over the C₃₊ hydrocarbon vapor, after exposure of theseparation membrane to liquid toluene and subsequent drying, that is atleast about 65% of a pre-exposure selectivity for carbon dioxide overthe C₃₊ hydrocarbon vapor, as measured pre- and post-exposure with atest gas mixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched incarbon dioxide compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in carbondioxide compared to the gas mixture.

The process differs from previous carbon dioxide/methane separationprocesses in the nature of the membrane that is used. The membranes are,as described above, able to maintain useful separation properties in thepresence of C₃₊ hydrocarbon vapor at high partial pressure, and able torecover from accidental exposure to liquid hydrocarbons. The membranesare also able to withstand high partial pressures of carbon dioxide.

The process of the invention typically provides a selectivity, inmixtures containing multiple hydrocarbons including a C₃₊ hydrocarbon,for carbon dioxide over methane of at least about 5, even at high carbondioxide activity. Frequently, the carbon dioxide/methane selectivityachieved is 10 or more, and may be as much as 15 or more, even in thepresence of significant concentrations of C₂₊ hydrocarbons.

In a different aspect, the invention is a process for separating notinorganic gases or vapors from organic gases or vapors, but organicgases or vapors from one another. In this aspect, the process of theinvention, again with reference to FIG. 1, involves running a streamcontaining a mixture of organic compounds, 1, across the feed side of amembrane separation unit, 2, containing a membrane as defined above, 3,that is selectively permeable to a first organic compound over a secondorganic compound in the stream. The first hydrocarbon is concentrated inthe permeate stream, 5; the residue stream, 4, is thus correspondinglydepleted of that hydrocarbon.

In this aspect, therefore, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising first and second organic componentsacross the feed side of a separation membrane having a feed side and apermeate side, the membrane having a selective layer comprising apolymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor the first organic component over the second organic component afterexposure of the separation membrane to liquid toluene and subsequentdrying, that is at least about 65% of a pre-exposure selectivity for thefirst organic component over the second organic component as measuredpre- and post-exposure with a test gas mixture of the same compositionand under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched in thefirst organic component compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in thefirst organic component compared to the gas mixture.

The process differs from previous hydrocarbon/hydrocarbon separationprocesses in the nature of the membrane that is used. The membranes are,as described above, more resistant to plasticization by hydrocarbonsthan prior art membranes, and are able to recover from accidentalexposure to liquid hydrocarbons.

The process of the invention may be used for diverse separations oforganic components, including, but not limited to, separation of methanefrom C₃₊ hydrocarbons, separation of olefins from paraffins; andseparation of isomers, such as n-butane from iso-butane. As it relatesto the separation of methane from other hydrocarbons, the inventionincludes the following steps:

(a) passing a gas mixture comprising methane and a C₃₊ hydrocarbon vaporacross the feed side of a separation membrane having a feed side and apermeate side, the membrane having a selective layer comprising apolymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor methane over the C₃₊ hydrocarbon vapor, after exposure of theseparation membrane to liquid toluene and subsequent drying, that is atleast about 65% of a pre-exposure selectivity for methane over the C₃₊hydrocarbon vapor, as measured pre- and post-exposure with a test gasmixture of the same composition and under like conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched inmethane compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted in methanecompared to the gas mixture.

In this aspect, the invention can be used in natural gas processing, forexample, to lower the Btu value and dewpoint of gas that initiallycontains excess C₃₊ hydrocarbons. The invention may also be used toremove the lightest hydrocarbons, specifically methane and ethane, fromprocess streams, to prevent their build up in a reactor loop, forexample.

The process of the invention typically provides a selectivity formethane over C₃₊ hydrocarbons, such as propane, butane or heavier, inmixtures containing multiple hydrocarbons including the C₃₊ hydrocarbon,of at least about 4 or 5, and in many cases, at least about 8.Frequently, the selectivity achieved is 10 or more, and may be as muchas 15 or more, even in the presence of significant concentrations of C₃₊hydrocarbons.

As it relates to the separation of olefins from paraffins, the inventionis particularly useful for separating propylene from propane. Suchmixtures are found as olefin manufacturing effluent streams, and invarious petrochemical plant streams, for example.

The process involves running a stream comprising propylene and propaneacross the feed side of a membrane that is selectively permeable topropylene. The propylene is concentrated in the permeate stream; theresidue stream is thus correspondingly depleted of propylene.

In a basic embodiment, the process of the invention includes thefollowing steps:

(a) passing a gas mixture comprising propylene and propane across thefeed side of a separation membrane having a feed side and a permeateside, the membrane having a selective layer comprising a polymer having:

(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;

(ii) a fractional free volume no greater than about 0.3; and

(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor propylene over propane after exposure of the separation membrane toliquid toluene and subsequent drying, that is at least about 65% of apre-exposure selectivity for propylene over propane as measured pre- andpost-exposure with a test gas mixture of the same composition and underlike conditions;

(b) providing a driving force for transmembrane permeation;

(c) withdrawing from the permeate side a permeate stream enriched inpropylene compared to the gas mixture;

(d) withdrawing from the feed side a residue stream depleted inpropylene compared to the gas mixture.

The process typically provides a propylene/propane selectivity of atleast about 2.5, more preferably at least about 3, which can besustained, even with streams composed entirely of C₃₊ hydrocarbons, overa range of pressures.

Since the membranes used in the invention are selective for both olefinsand hydrogen over paraffins, the membrane separation step may be used,where both are present, to produce a permeate enriched in both olefinsand hydrogen, leaving a residue stream enriched in paraffins. Theolefins in the permeate stream may then be separated from the hydrogento deliver product streams of each.

Optionally, the processes of the invention already discussed may includeother separation steps used in conjunction with the defined membraneseparation process. Examples of such separation steps includeadsorption, absorption, condensation, and distillation. The otherseparation steps may be carried out upstream, downstream or both of themembrane separation step, that is, with reference to FIG. 1 on any ofstreams 1, 4 and 5. Specific examples of such processes are discussed inmore detail in copending serial number 09/574,420, entitled “GasSeparation Using Organic-Vapor-Resistant Membranes” incorporated hereinby reference in its entirety.

The invention is now illustrated in further detail by specific examples.These examples are intended to further clarify the invention, and arenot intended to limit the scope in any way.

EXAMPLES Example 1 Membrane Making

Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared using thefollowing coating solutions:

1 wt % copolymer solution of 40% tetrafluoroethylene/60%2,2,4-trifluoro-5-trifluorometoxy-1,3-dioxole (Hyflon® AD60), (Ausimont,Italy), in a perfluorinated solvent (Fluorinert FC-84), (3M, St. Paul,MN).

1 wt % copolymer solution of 20% tetrafluoroethylene/80%2,2,4-trifluoro-5-trifluorometoxy-1,3-dioxole (Hyflon® AD80), (Ausimont,Italy), in FC-84 solvent.

1 wt % polyperfluoro (alkenyl vinyl ether) (Cytop®), (Asahi Glass,Japan), in FC-84 solvent.

The support membranes were dip-coated in a solution of one of the threeselective polymer solutions at 1 ft/min coating speed, then dried in anoven at 60° C. for 10 minutes. The resulting membranes had a selectivelayer thickness ranging from 0.2-0.5 μm. Samples of each finishedcomposite membrane were cut into 12.6 cm² stamps and tested in apermeation test-cell apparatus with pure gases at 35° C. feedtemperature and 65 psia feed pressure. During each test, the feed,permeate, and residue compositions were analyzed by gas chromatography(GC). The gas fluxes of the membranes were measured, and theselectivities were calculated. Table 1 summarizes the fluxes and Table 2summarizes the selectivities of the composite membranes, calculated asthe ratio of the pure gas fluxes.

TABLE 1 Pure-Gas Pressure-Normalized Flux (GPU) Gas Hyflon ® AD60Hyflon ® AD80 Cytop ® Nitrogen 52 184 34 Oxygen 180 574 130 Helium 1,3601,850 1,270 Hydrogen 790 2,040 620 Argon 85.4 289 56 Carbon Dioxide 433— 300 Methane 17.6 65.8 11 Ethane 4.5 18.8 3 Ethylen 9.8 35.9 5.7Propane 1.1 — 3.4 Propylene 5.1 25.6 — CF₄ 0.94 3.38 0.48 NF₃ 10.3 38.85.7 1 GPU = 1 × 10⁻⁶ cm³ (STP)/cm² · s · cmHg

TABLE 2 Selectivity (−) Gas Pair Hyflon ® AD60 Hyflon ® AD80 Cytop ®N₂/CF₄ 55 58 71 O₂/N₂ 3.5 3.1 3.8 N₂/CH₄ 2.9 2.8 3.2 He/H₂ 1.7 0.91 2.0Ar/CH₄ 4.8 4.4 5.3 Ar/C₂H₄ 8.7 8.0 9.7 CO₂/CH₄ 26 — 28 H₂/CH₄ 45 31 59N₂/C₂H₄ 5.3 5.1 6.0 N₂/C₂H₆ 10 7.2 —

Example 2 Mixed-Gas Argon/Ethylene Permeation Properties

Membranes were prepared and membrane stamps were subjected to permeationexperiments using the same general procedure as in Example 1. Thetemperature was 23° C., the feed pressure was 165 psia, and the feed gasmixture contained approximately 9% argon, 67% methane and 24% ethylene.The pressure-normalized fluxes of argon and ethylene were measured, andthe argon/ethylene selectivities were calculated. The results are shownin Table 3.

TABLE 3 Mixed-Gas Pressure-Normalized Flux (GPU) Ar/C₂H₄ Membrane ArC₂H₄ Selectivity (−) Hyflon ® AD60 88 11.9 7.4 Hyflon ® AD80 265 42.76.2 Cytop ® 51 5.8 8.8

Example 3 Mixed-Gas Nitrogen/Ethylene Permeation Properties

Membranes were prepared and membrane stamps were subjected to permeationexperiments using the same general procedure as in Example 1. Thetemperature was 23° C., the feed pressure was 165 psia, and the feed gasmixture contained 80% nitrogen and 20% ethylene. The pressure-normalizedfluxes of nitrogen and ethylene were measured, and the nitrogen/ethyleneselectivities were calculated. The results are shown in Table 4.

TABLE 4 Mixed-Gas Pressure-Normalized Flux (GPU) N₂/C₂H₄ Membrane N₂C₂H₄ Selectivity (−) Hyflon ® AD60 53 11 4.8 Hyflon ® AD80 184 41.8 4.4Cytop ® 31 5.3 5.8

Example 4 Mixed-Gas Carbon Dioxide/Methane Permeation Properties

Membranes were prepared and membrane stamps were subjected to permeationexperiments using the same general procedure as in Example 1. Thetemperature was 22° C., the feed pressure was 115 psia, and the feed gasmixture contained 65% carbon dioxide, 25% methane and 10% propane. Thepressure-normalized fluxes of carbon dioxide and methane were measured,and the carbon dioxide/methane selectivities were calculated. Theresults are shown in Table 5.

TABLE 5 Mixed-Gas Pressure-Normalized Flux (GPU) CO₂/CH₄ Membrane CO₂CH₄ Selectivity (−) Hyflon ® AD60 460 27 17 Hyflon ® AD80 1,620 125 13Cytop ® 128 5.8 22

Example 5 Binary-Mixed-Gas Carbon Dioxide/Methane Permeation Properties

A Hyflon® AD60 membrane was prepared and subjected to permeationexperiments using the same general procedure as in Example 1. Thetemperatures ranged from −20 to 20° C., the feed pressures ranged from115 to 415 psia, and the feed gas mixture contained 70% carbon dioxideand 30% methane. The pressure-normalized fluxes of carbon dioxide andmethane were measured, and the carbon dioxide/methane selectivities werecalculated. The results are shown in Table 6.

TABLE 6 Mixed-Gas Pressure-Normalized Temperature Pressure Flux (GPU)CO₂/CH₄ (° C.) (psia) CO₂ CH₄ Selectivity (−)  20 115 89 5.2 17 −20 11592 2.6 36 −20 215 113 3.8 29 −20 315 279 13 21 −20 415 1,420 167 8.5

As can be seen from the table, the membranes retained useful carbondioxide/methane selectivities over the test range. At −20° C., thesaturation vapor pressure of carbon dioxide is 285 psia. Under theextreme conditions of low temperature (−20° C.) combined with highpressure (415 psia) of the test, carbon dioxide partial pressure reached290 psia, i.e., saturation. Even when the gas mixture was saturated withcarbon dioxide, the membranes withstood plasticization by carbon dioxidewell enough to retain the carbon dioxide/methane selectivity at a usablelevel.

Example 6 Solvent Resistance of Hyflon® AD60 compared to Polysulfone

Experiments were carried out to determine the stability of a Hyflon®AD60 membrane in the presence of hydrocarbon solvents. Samples of aHyflon® AD60 membrane were tested in a permeation test-cell as inExample 1. The fluxes were measured and the selectivities calculated.The membrane stamps were then immersed in liquid toluene or hexane.After one week, the membranes were removed from the hydrocarbon liquid,dried at ambient temperature, and retested in the gas permeationtest-cell. A polysulfone (PSF) asymmetric membrane, typically used inhydrogen separation processes, was also tested for comparison. Thepermeation properties of the Hyflon® AD60 and polysulfone membranesbefore and after exposure to the hydrocarbon solvent are summarized inTable 7.

TABLE 7 Initial Flux Initial Post-Toluene Post-Toluene (GPU) Selectivity(−) Flux (GPU) Selectivity (−) Membrane N₂ H₂ O₂/N₂  H₂/CH₄ N₂ H₂ O₂/N₂ H₂/CH₄ Hyflon ® 30 350 3.1 25 41 477 3.1 26 PSF 1.2 — 5.6 — DissolvedInitial Flux Initial Post-Hexane Post-Hexane (GPU) Selectivity (−) Flux(GPU) Selectivity (−) Membrane N₂ H₂ O₂/N₂  H₂/CH₄ N₂ H₂ O₂/N₂  H₂/CH₄Hyflon ® 31 350 3.0 24 41 480 3.1 27 PSF 0.6  50 6.8 99 1.6 87 5.9 48

As can be seen, the polysulfone membranes could not withstand exposureto toluene, and their hydrogen/methane selectivity declined by halfafter exposure to hexane. In contrast, the dioxole copolymer Hyflon®membranes, although they exhibited higher fluxes for all gases for whichthey were tested after soaking in liquid hydrocarbons, retained theirhydrogen/methane selectivity.

Examples 7-10 Comparative Examples with Teflon® AF 2400 CompositeMembranes—Not in Accordance with the Invention Example 7 Membrane Making

Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared bydip-coating the support membranes three times in a solution of 1 wt %2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylenecopolymer [Teflon® AF2400] solution in FC-84 solvent at 1 ft/min coatingspeed, then dried in an oven at 60° C. for 10 minutes. The resultingmembranes had a selective layer thickness of 4 μm. Samples of eachfinished composite membrane were cut into 12.6 cm² stamps and tested ina permeation test-cell apparatus with pure oxygen and nitrogen at 22° C.feed temperature and 65 psia feed pressure. During each test, the feed,permeate, and residue compositions were analyzed by gas chromatography(GC). The gas fluxes were measured, and the selectivities werecalculated. Table 8 summarizes the pressure-normalized fluxes andselectivities of the composite Teflon® AF membranes.

TABLE 8 Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) N₂ O₂O₂/N₂ 185 353 1.9

Example 8 Mixed-Gas Argon/Ethylene Permeation Properties

Membranes were prepared and subjected to permeation experiments usingthe same general procedure as in Example 7. The temperature was 22° C.,the feed pressure was 165 psia, and the feed gas mixture wasapproximately 8% argon, 65% methane and 27% ethylene. Thepressure-normalized fluxes of the gases were measured, and theselectivities were calculated. The results are shown in Table 9.

TABLE 9 Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) Ar CH₄C₂H₄ Ar/CH₄ Ar/C₂H₄ 232 159 158 1.5 1.5

As can be seen, the membranes are only slightly selective for argon overethylene. In contrast, Example 2 showed that the membranes of theinvention had exceptionally high argon/ethylene selectivities in therange of about 6 to 9.

Example 9 Mixed-Gas Nitrogen/Ethylene Permeation Properties

Membranes were prepared and subjected to permeation experiments usingthe same general procedure as in Example 7. The temperature was 22° C.,the feed pressure was 165 psia, and the feed gas mixture was 80%nitrogen and 20% ethylene. The pressure-normalized fluxes of nitrogenand ethylene were measured, and the nitrogen/ethylene selectivities werecalculated. The results are shown in Table 10.

TABLE 10 Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) N₂C₂H₄ N₂/C₂H₄ 177 159 1.1

The membrane was essentially unselective for nitrogen over ethylene. Incontrast, Example 3 showed selectivities of about 4 to 6 for nitrogenover ethylene for the membranes of the invention.

Example 10 Mixed-Gas Carbon Dioxide/Methane Permeation Properties

Membranes were prepared and subjected to permeation experiments usingthe same general procedure as in Example 7. The temperature was 22° C.,the feed pressure was 115 psia, and the feed gas mixture was 64% carbondioxide, 25% methane and 11% propane. The pressure-normalized fluxes ofthe gases were measured, and the selectivities were calculated. Theresults are shown in Table 11.

TABLE 11 Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) CO₂CH₄ C₃H₈ CO₂/CH₄ CO₂/C₃H₈ 831 175 95.7 4.8 8.7

In this case, the carbon dioxide/methane selectivity was only 4.8,compared with 13-22 in experiments under similar conditions with themembranes of the invention reported in Example 4.

Examples 11-13 Comparison of Pure-Gas Permeation Properties with Hyflon®AD and Teflon® AF2400 Membranes Example 11 Hyflon® AD60 Pure-GasPermeation Properties

Hyflon® AD60 membranes were prepared as in Example 1, except using apoly(etherimide) support layer. The resulting membranes were tested asin Example 1 with pure hydrogen, nitrogen, methane, ethane, propane, andn-butane at 35° C. at feed pressures ranging from 35 to 165 psia. Then-butane was tested only at 32 psia, which is nearly 70% of thesaturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 2. Thecalculated nitrogen/hydrocarbon selectivities are shown graphically inFIG. 3, and the calculated hydrogen/hydrocarbon selectivities are shownin FIG. 4.

As can be seen in FIG. 2, the hydrogen, nitrogen, and methane fluxesremained nearly constant across the range of pressures. The ethane fluxincreased from 6.9 GPU at 65 psia to 12.6 GPU at 165 psia, and thepropane flux increased from 1.4 GPU at 35 psia to 3.9 GPU at 125 psia,which is about 70% of the saturation vapor pressure (180 psia) ofpropane at 35° C. As shown in FIG. 3, the nitrogen/methane selectivityremained constant at approximately 2.3 across the range of pressures.The nitrogen/ethane selectivity decreased from 8.2 at 65 psia to 4.5 at165 psia, and the nitrogen/propane selectivity decreased from 42 at 35psia to 30 at 125 psia. Although the nitrogen/propane selectivitydecreased over the pressure range, the membrane remained nitrogenselective, at a useful selectivity, over the entire pressure range up tonear-saturation.

As shown in FIG. 4, the hydrogen/methane selectivity remained constantat approximately 29 across the range of pressures. The hydrogen/ethaneselectivity decreased slightly from 97 at 65 psia to 83 at 115 psia,then decreased further to 57 at 165 psia. The hydrogen/propaneselectivity was 230 at 115 psia.

Example 12 Hyflon® AD80Pure-Gas Permeation Properties

Hyflon® AD80membranes were prepared as in Example 1, except using apoly(etherimide) support layer. The resulting membranes were tested asin Example 1 with pure hydrogen, nitrogen, methane, ethane, propane, andn-butane at 35° C. at feed pressures ranging from 35 to 165 psia. Then-butane was tested only at 32 psia, which is nearly 70% of thesaturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 5. Thecalculated hydrogen/hydrocarbon selectivities are shown graphically inFIG. 6 and the calculated nitrogen/hydrocarbon selectivities are shownin FIG. 7.

As can be seen in FIG. 5, the hydrogen, nitrogen, methane and ethanefluxes remained nearly constant across the range of pressures. Thepropane flux increased from 3 GPU at 35 psia to 6.6 GPU at 120 psia. Asshown in FIG. 6, the hydrogen/methane and hydrogen/ethane selectivitiesremained constant at approximately 20 and 44, respectively, across therange of pressures. The hydrogen/propane selectivity decreased from 140at 35 psia to 66 at 120 psia. Thus, as in the previous example, themembranes retained useful hydrogen/hydrocarbon selectivity, even atclose to hydrocarbon saturation. The hydrogen/n-butane selectivity was373.

As shown in FIG. 7, the nitrogen/methane and nitrogen/ethaneselectivities remained nearly constant at approximately 1.7 and 3.8,respectively, across the range of pressures. The nitrogen/propaneselectivity was 5.9 at 115 psia.

Example 13 Teflon® AF2400 Pure-Gas Permeation Properties—Not inAccordance with the Invention

Teflon® AF2400 membranes were prepared as in Example 7, except using apoly(etherimide) support layer. The resulting membranes were tested asin Example 7 with pure hydrogen, nitrogen, methane, ethane, propane, andn-butane at 35° C. at pressures ranging from 17 to 165 psia. Then-butane was tested only up to 31 psia, 31 psia being about 65% of thesaturation vapor pressure of n-butane at 35° C. The measuredpressure-normalized gas fluxes are shown graphically in FIG. 8. Thecalculated nitrogen/hydrocarbon and hydrogen/hydrocarbon selectivitiesare shown graphically in FIGS. 9 and 10, respectively.

As can be seen in FIG. 8, the hydrogen, nitrogen, methane, and ethanefluxes remained nearly constant across the range of pressures. Thepropane flux increased nearly five-fold from 268 GPU at 35 psia to 1,310GPU at 120 psia, and the n-butane flux increased from 400 GPU at 17 psiato 1,110 GPU at 31 psia.

As shown in FIG. 9, the nitrogen/methane and nitrogen/ethaneselectivities were all low and remained constant at approximately 1.1and 1.6, respectively, across the range of pressures. Thenitrogen/propane selectivity decreased from 3.0 at 35 psia to 1.0 at 95psia, about half the saturation vapor pressure of propane at 35° C.,then to 0.6 at 120 psia. In other words, the membrane selectivity wasinitially low, and the membrane lost its nitrogen/propane selectivitycompletely by about 50% saturation and became hydrocarbon-selective asthe pressure increased towards the propane saturation vapor pressure.Likewise, the nitrogen/n-butane selectivity decreased from 2 at 17 psiato 1 at 27 psia, then to 0.7 at 31 psia, again indicating that themembrane had become hydrocarbon-selective as the pressure increased.

As shown in FIG. 10, the hydrogen/methane selectivity remained constantat approximately 4.4 across the range of pressures. The hydrogen/ethaneselectivity decreased slightly from 6.4 at 65 psia to 5.9 at 115 psia.The hydrogen/propane selectivity decreased from 11.6 at 35 psia to 2.4at 120 psia, indicating that the Teflon® AF was being plasticized by thepropane. The selectivity declined to about 5, less than half itsoriginal value, at a pressure of about 75 psia, which is only about 40%of the 180 psia saturation vapor pressure of propane at 35° C. Likewise,the hydrogen/n-butane selectivity decreased from 7.8 at 17 psia to 2.8at 31 psia, again indicating that the material had plasticized and lostits hydrogen-selective capability in the presence of C₃₊ hydrocarbons.

Examples 14-15 Hyflon® AD60 Multicomponent Mixed-Gas PermeationProperties as a Function of Pressure Example 14

Hyflon® AD60 membranes were prepared as in Example 11 above and weretested with a gas mixture containing approximately 63% carbon dioxide,27% methane, and 10% propane at 22° C. at feed pressures ranging from115 to 415 psia. The saturation vapor pressure of the gas mixture isabout 915 psia; thus, at 415 psia, the mixture was about 45% saturated.The measured pressure normalized gas fluxes are shown graphically inFIG. 11. The calculated carbon dioxide/hydrocarbon selectivities areshown graphically in FIG. 12.

As can be seen in FIG. 11, the fluxes all increased across the range ofpressures. The carbon dioxide flux increased from 46.5 GPU to 136 GPU.The methane flux increased from 3.1 GPU to 11.6 GPU. The propane fluxincreased from 0.3 GPU to 2.0 GPU. As shown in FIG. 12, the carbondioxide/methane selectivity decreased only slightly from 15 to 12 acrossthe range of pressures. The carbon dioxide/propane selectivity decreasedfrom 152 to 68.

Example 15

Hyflon® AD60 membranes were prepared as in Example 11 above and weretested with a gas mixture containing approximately 42% hydrogen, 20%methane, 25% ethane, 11% propane, and 1.4% n-butane at 25° C. at feedpressures ranging from 115 to 415 psia. The saturation vapor pressure ofthe gas mixture was about 1,130 psia; thus, at 415 psia, the mixture wasabout 37% saturated. The measured pressure-normalized gas fluxes areshown graphically in FIG. 13. The calculated hydrogen/hydrocarbonselectivities are shown graphically in FIG. 14.

As can be seen in FIG. 13, the fluxes of hydrogen, methane, ethane, andpropane increased slightly across the range of pressures. The n-butaneflux decreased slightly from 0.23 GPU at 115 psia to 0.20 GPU at 415psia. As shown in FIG. 14, the hydrogen/methane, hydrogen/ethane, andhydrogen/propane selectivities decreased slightly across the range ofpressures. The hydrogen/n-butane selectivity appeared to increase from280 to 328 as the feed pressure increased, but this apparent increase iswithin the range of experimental error.

Examples 16-18 Comparison of Methane/n-Butane Permeation Properties withHyflon® AD and Teflon® AF2400 Membranes Example 16 Methane/n-ButanePermeation Properties with Hyflon® AD60 Membranes

Hyflon® AD60 membranes were prepared and membrane stamps were subjectedto permeation experiments using the same general procedure as inExample 1. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. The saturation vapor pressure of n-butaneat 21° C. is about 31 psia; thus, at the highest n-butane concentration(8%), the gas mixture was about 25% saturated. The pressure-normalizedfluxes of methane and n-butane were measured, and the methane/n-butaneselectivities at the varying n-butane concentrations were calculated.The results are shown in Table 12.

TABLE 12 n-C₄H₁₀ Mixed-Gas Pressure- CH₄/n-C₄H₁₀ ConcentrationNormalized Flux (GPU) Selectivity (%) CH₄ n-C₄H₁₀ (−) 2 11.4 2.6 4.4 411.0 2.4 4.5 6 10.6 2.5 4.2 8 10.5 2.6 4.1

FIGS. 15 and 16 are graphs showing the measured pressure-normalizedfluxes and the calculated selectivities, respectively. As can be seen,the fluxes and selectivities remain nearly constant over the range ofn-butane concentrations.

Example 17 Methane/n-Butane Permeation Properties with Hyflon® AD80Membranes

Hyflon® AD80 membranes were prepared and membrane stamps were subjectedto permeation experiments using the same general procedure as inExample 1. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. Again, at the highest n-butaneconcentration (8%), the gas mixture was about 25% saturated. Thepressure-normalized fluxes of methane and n-butane were measured, andthe methane/n-butane selectivities at the varying n-butaneconcentrations were calculated. The results are shown in Table 13.

TABLE 13 n-C₄H₁₀ Mixed-Gas Pressure- CH₄/n-C₄H₁₀ ConcentrationNormalized Flux (GPU) Selectivity (%) CH₄ n-C₄H₁₀ (−) 2 31 4.8 6.4 4 324.7 6.7 6 29 4.7 6.3 8 31 4.7 6.6

FIGS. 17 and 18 are graphs showing the measured pressure-normalizedfluxes and the calculated selectivities, respectively. As can be seen,the fluxes and selectivities remain nearly constant over the range ofn-butane concentrations.

Example 18 Methane/n-Butane Permeation Properties with Teflon® AF2400Membranes—Not in Accordance with the Invention

Teflon® AF2400 membranes were prepared and membrane stamps weresubjected to permeation experiments using the same general procedure asin Example 7. The temperature was 21° C., the pressure was 115 psia, andthe feed gas mixture contained n-butane in varying concentrations from2-8% and the balance methane. Again, at the highest n-butaneconcentration (8%), the gas mixture was about 25% saturated. Thepressure-normalized fluxes of methane and n-butane were measured, andthe methane/n-butane selectivities at the varying n-butaneconcentrations were calculated. The results are shown in Table 14.

TABLE 14 n-C₄H₁₀ Mixed-Gas Pressure- CH₄/n-C₄H₁₀ ConcentrationNormalized Flux (GPU) Selectivity (%) CH₄ n-C₄H₁₀ (−) 2 103 71 1.4 4 12282 1.5 6  92 80 1.2 8 112 103  1.1

FIGS. 19 and 20 are graphs showing the measured pressure-normalizedfluxes and the calculated selectivities, respectively. As can be seen inFIG. 20, the membranes are only marginally selective for methane overn-butane, and decreasingly so at higher n-butane concentrations.

Examples 19-21 Mixed-Gas Permeation Properties in Modules Example 19Hyflon® AD60 Membrane Module Permeation Properties at 20° C.

Hyflon® AD60 membranes were prepared as in Example 11. The resultingmembranes were rolled into a spiral-wound module, which was tested in amodule test apparatus at 20° C. at varying pressures. The feed gasmixture was 65% methane, 10% ethane, 5% propane, and 20% carbon dioxide.The saturation vapor pressure of this gas mixture was calculated to beapproximately 1,150 psia. The pressure-normalized gas fluxes weremeasured and the selectivities calculated. The results are shown inTable 15.

TABLE 15 Pressure-Normalized Flux CO₂/CH₄ CO₂/C₃H₈ CH₄/C₃H₈ Pressure(GPU) Selectivity Selectivity Selectivity (psia) CH₄ C₂H₆ C₃H₈ CO₂ (−)(−) (−) 213 8.1 3.4 1.6 135 16.6 84.5 5.1 315 7.9 3.4 1.7 117 14.8 69.04.6 414 9.2 4.1 2.0 123 13.4 61.6 4.6 515 11.1 4.9 2.3 132 11.8 57.3 4.8615 14.4 6.5 2.6 148 10.2 56.7 5.5 715 16.0 7.4 3.0 146 9.1 48.8 4.6 81518.8 8.9 3.5 148 7.9 42.4 5.4 915 22.8 11.5 4.4 152 6.7 34.5 5.2 1,015  29.1 15.8 7.0 146 5.0 20.8 4.1

As can be seen, the carbon dioxide flux remained relatively stableacross the range of pressures. The methane and propane fluxes increased3- to 4-fold with increasing pressure, resulting in the carbondioxide/methane and carbon dioxide/propane selectivities decreasing withincreasing pressure. However, even at 615 psia, at greater than 50%saturation, the membrane maintained a carbon dioxide/methane selectivityof 10.

Example 20 Hyflon® AD60 Membrane Module Permeation Properties at 0° C.

The experiment of Example 19 was repeated, except at 0° C. at varyingpressures. The feed gas mixture was 65% methane, 10% ethane, 5% propane,and 20% carbon dioxide. At this low temperature, the saturation vaporpressure of the gas mixture was calculated to be approximately 915 psia.The pressure-normalized gas fluxes were measured and the selectivitiescalculated. The results are shown in Table 16.

TABLE 16 Pressure-Normalized Flux CO₂/CH₄ CO₂/C₃H₈ CH₄/C₃H₈ Pressure(GPU) Selectivity Selectivity Selectivity (psia) CH₄ C₂H₆ C₃H₈ CO₂ (−)(−) (−) 213 5.3 2.6 1.7 116 21.6 67.9 3.1 315 5.1 2.5 1.6 95.2 18.8 59.53.2 414 6.5 3.3 1.8 108 16.7 59.9 3.6 515 7.4 3.7 2.1 120 16.2 57.0 3.5615 12.5 6.7 3.2 151 12.0 47.2 3.9 715 17.1 10.0 4.2 170 10.0 40.6 4.1815 22.5 13.8 6.9 184 8.1 26.6 3.3 915 45.2 36.6 20.5 222 4.9 10.8 2.21,015   54.5 43.7 23.6 224 4.1 9.5 2.3

As can be seen, the carbon dioxide flux nearly doubled across the rangeof pressures. The methane and propane fluxes increased 10- to 14-foldwith increasing pressure, resulting in the carbon dioxide/methane andcarbon dioxide/propane selectivities again decreasing with increasingpressure. However, even at 715 psia, at nearly 80% saturation, themembrane maintained a carbon dioxide/methane selectivity of 10.

Example 21 Effect of Temperature and Hydrocarbon Saturation onSelectivity

Based on the data from Examples 19 and 20, the carbon dioxide/methaneselectivity was calculated as a function of temperature and percentsaturation, expressed as the ratio of pressure to saturated vaporpressure or critical pressure. The results are shown in FIG. 21. As canbe seen, selectivity declines with increasing saturation, but remainsacceptable even at high saturation levels.

Examples 22-25 Effect of Carbon Dioxide on Plasticization of Hyflon®Membranes Example 22 Hyflon® AD60 Membrane Permeation Properties at 20°C. at Varying Pressures

A Hyflon® AD60 membrane was made and a membrane stamp was tested as inExample 11 at 20° C. at varying pressures. The feed gas contained 30%methane and 70% carbon dioxide. The pressure-normalized gas fluxes weremeasured and the selectivities calculated. The results are shown inFIGS. 22 and 23, respectively. As can be seen in FIG. 22, the carbondioxide flux increased only slightly from 63 GPU at 115 psia to 76 GPUat 415 psia. FIG. 23 shows that, as a result, the carbon dioxide/methaneselectivity decreased only slightly from 15 at 15 psia to 12 at 415psia.

Example 23 Hyflon® AD60 Membrane Permeation Properties at −20° C. atVarying Pressures

The experiment of Example 22 was repeated, except at −20° C. at varyingpressures. At −20° C., the saturation vapor pressure of carbon dioxideis about 285 psia. The gas fluxes were measured and the selectivitiescalculated. The results are shown in FIGS. 24 and 25, respectively. Ascan be seen in FIG. 24, the carbon dioxide flux increased only slightlyfrom 94 GPU at 115 psia to 113 GPU at 215 psia. The flux then increasedto 280 GPU at 315 psia, and then sharply to 1,430 GPU at 415 psia,indicating that the membrane had plasticized under the extremeconditions of low temperature and high pressure. FIG. 25 shows that, asa result, the carbon dioxide/methane selectivity decreased from 36 at115 psia to 9 at 415 psia.

Example 24 Reversal of Plasticization in Hyflon® AD60 Membrane Module

The membrane stamps used in the experiments of Examples 22 and 23 hadbeen tested for their pure-gas permeation properties before they wereused under the high-pressure, low-temperature conditions that causedthem to become severely plasticized. After the plasticizationexperiments had been completed, the membranes were retested with thesame set of pure gases. The results of the tests are shown in Table 17.

TABLE 17 Pressure-Normalized Flux (GPU) Selectivity (−) Before TestAfter Test Before Test After Test O₂ N₂ CO₂ CH₄ O₂ N₂ CO₂ CH₄ O₂/N₂CO₂/CH₄ O₂/N₂ CO₂/CH₄ 55.0 17.2 135 4.1 47.8 14.5 137 6.5 3.2 19.0 3.321.0

As can be seen, the pre- and post-plasticization-test permeationproperties are essentially the same, within the limits of experimentalerror. The Hyflon® membranes were able to regain their originalpermeation properties. Thus, the plasticization did not causeirreversible damage.

Example 25 Selectivity at Varying Saturation Levels and PartialPressures

Based on the data of Examples 22 and 23, the carbon dioxide/methaneselectivity was calculated as a function of percent saturation,expressed as the ratio of pressure to saturation vapor pressure. Theresults are shown graphically in FIG. 26. As can be seen, at 20° C., theselectivity decreased slightly, from 15 to 12, over the saturationrange. At −20° C., the selectivity decreased sharply from 36 at about30% saturation to 9 as the gas mixture approached saturation.

Example 26 Mixed-Gas Nitrogen/Propylene Permeation Properties withHyflon® and Cytop® Membranes

Hyflon® and Cytop® membranes were prepared and membrane stamps weresubjected to permeation experiments using the same general procedure asin Example 1. The temperature was 23° C., the pressure was 165 psia, andthe feed gas mixture contained 90% nitrogen and 10% propylene. Thesaturation vapor pressure of propylene at 23° C. is about 160 psia, sothe gas mixture was only about 10% saturated. The pressure-normalizedfluxes of nitrogen and propylene were measured, and thenitrogen/propylene selectivities were calculated. The results are shownin Table 18.

TABLE 18 Mixed-Gas Pressure-Normalized Flux (GPU) N₂/C₃H₆ Membrane N₂C₃H₆ Selectivity (−) Hyflon ® AD60 50 4.5 11 Hyflon ® AD80 167 17.8 9.4Cytop ® 30 2.3 13

Example 27 Comparative Example of Mixed-Gas Nitrogen/PropylenePermeation Properties with Teflon® AF 2400 Membranes—Not in Accordancewith the Invention

Teflon® AF 2400 membranes were prepared and subjected to permeationexperiments using the same general procedure as in Example 7. Thetemperature was 22° C., the pressure was 165 psia, and the feed gasmixture was 90% nitrogen and 10% propylene. Again, the saturation vaporpressure of propylene at 22° C. is about 160 psia, so the gas mixturewas only about 10% saturated. The pressure-normalized fluxes of nitrogenand propylene were measured, and the nitrogen/propylene selectivity wascalculated. The results are shown in Table 19.

TABLE 19 Mixed-Gas Pressure-Normalized Flux (GPU) Selectivity (−) N₂C₃H₆ N₂/C₃H₆ 151 176 0.85

As can be seen by comparing Examples 26 and 27, the membranes of theinvention provided exceptionally high nitrogen/propylene selectivitiesthat ranged from about 9 to 13. In contrast, the Teflon® AF 2400membranes were essentially unselective, but slightly favored permeationof ethylene over nitrogen.

Examples 28-31 Comparison of Olefin/Paraffin Separation using Hyflon®AD60 Membranes (According to the Invention) and Polyimide Membranes (Notin Accordance with the Invention) Example 28 Olefin/Paraffin SeparationProperties with a Hyflon® AD60 Membrane Module

The spiral-wound Hyflon® AD60 module prepared in Example 19 above wastested with a gas mixture comprising approximately 60% propylene and 40%propane at 30° C. at pressures ranging from 65 to 165 psia. Thesaturation vapor pressure of the gas mixture at 30° C. is about 177psia; thus, at the highest pressure tested, the gas mixture was nearsaturation. The measured pressure-normalized propylene fluxes are showngraphically in FIG. 27. The calculated propylene/propane selectivitiesare shown graphically in FIG. 28.

As can be seen in FIG. 27, the propylene flux increased from about 6 GPUat 65 psia, to about 9 GPU at 115 psia, and to about 19 GPU at 165 psia.As shown in FIG. 28, the propylene/propane selectivities remainedessentially constant in the range 3.0 to 3.3 across the range ofpressures.

Another spiral-wound module prepared as in Example 19 was subjected tothe same tests and yielded very similar results. The propylene andpropane fluxes are shown in FIG. 29 as a function percent saturation,expressed as the ratio of pressure to saturation vapor pressure. As canbe seen, the fluxes increased only gradually, and remained essentiallystable up to about 80% saturation.

Example 29 Olefin/Paraffin Separation Properties with a PolyimideMembrane Module

Asymmetric, microporous poly(vinylidene fluoride) [PVDF] supportmembranes were prepared. Composite membranes were prepared using acoating solution of 1 wt % of the polyimidepoly(3,4,3′,4′-biphenyltetracarboxylicdianhydride-2,4,6-m-phenylenediamine) [BPDA-TMPD] in ahexafluoropropanol/chloroform solvent.

The support membranes were dip-coated in the BPDA-TMPD solution at 0.5ft/min coating speed, then dried in an oven at 100° C. for 30 minutes.The resulting membranes had a selective layer approximately 0.2 μmthick. The membranes were rolled into a spiral-wound module, which wastested in a module test apparatus with pure oxygen and nitrogen todetermine the integrity of the membrane. The module was then subjectedto experiments at 16° C. and 60° C. at 75-95 psia. The feed gas mixturewas approximately 60% propylene and 40% propane. The saturation vaporpressure of the gas mixture at 16° C. is about 125 psia, and at 60° C.is about 350 psia; thus, at the highest pressure tested, 95 psia, thegas mixture was about 76% saturated at 16° C., and about 27% saturatedat 60° C. The pressure-normalized gas fluxes were measured. The resultsare presented in FIGS. 30 and 31, graphs of the propylene and propanefluxes, respectively, as a function of feed pressure at the twotemperatures. For both gases at the lower temperature, the fluxesincreased sharply above 85 psia, that is, above about 68% saturation.Because the increases in the gas fluxes were proportionate, thecalculated selectivities remained between about 2.5 to 4 across thepressure range.

Example 30 Recovery of Separation Properties in a Polyimide MembraneModule

The BPDA-TMPD membrane module used in Example 29 was operatedcontinuously for eight days in the module test apparatus with a 60%propylene/40% propane gas mixture at 65 psia and 30° C. At the end ofthis time, the module was retested with pure oxygen and nitrogen. ABPDA-TMPD membrane stamp, which was also subjected to the tests ofExample 29, was tested with pure oxygen and nitrogen for comparison. Thegas fluxes were measured and the selectivities calculated. The resultsare summarized in Table 20.

TABLE 20 Pressure-Normalized Flux (GPU) Selectivity (−) Before TestAfter Test Before Test After Test Configuration O₂ N₂ O₂ N₂ O₂/N₂ O₂/N₂Stamp 22.9 4.3 5.5 2.0 5.3 2.8 Module 6.0 1.1 4.7 2.3 5.6 2.0

The stamp and module oxygen/nitrogen selectivities were comparable priorto the tests. After the tests, the oxygen/nitrogen selectivitiesdecreased significantly in both configurations. The results indicatethat the damage done to the polyimide membranes as a result of long-termexposure to the hydrocarbons was irreversible.

Examples 31-33 Comparison of Nitrogen/VOC Permeation Properties usingHyflon® AD60 Membranes (According to the Invention) and PolyimideMembranes (Not in Accordance with the Invention) Example 31Nitrogen/Dimethylethylamine Mixed-Gas Separation Properties

Hyflon® AD60 membranes were prepared as in Example 11, and membranestamps were subjected to permeation experiments using the same generalprocedure as in Example 1. The temperature was 21° C., the pressure was65 psia, and the feed gas mixture contained dimethylethylamine (DMEA) invarying concentrations from 3.2-16.6% (16.6% is saturation) and thebalance nitrogen. The pressure-normalized fluxes of DMEA and nitrogenwere measured, and the nitrogen/DMEA selectivities at the varying DMEAconcentrations were calculated. The results are shown in Table 21.

TABLE 21 DMEA Mixed-Gas Pressure- N₂/DMEA Concentration Normalized Flux(GPU) Selectivity (%) N₂ DMEA (−) 3.2 10.4 0.06 163 7.5 9.5 0.08 11513.5 9.1 0.13 73 16.6 8.8 0.15 60

FIGS. 32 and 33 are graphs showing the measured pressure-normalizedfluxes and the calculated selectivities, respectively. As can be seen inFIG. 32, the nitrogen flux remained nearly constant over the range ofDMEA concentrations; the DMEA flux increased as the DMEA concentrationincreased. As a result, the nitrogen/DMEA selectivity decreased as theDMEA concentration increased, as shown in FIG. 33. The membranesretained acceptable flux and selectivity even in the presence of DMEA atsaturation.

Example 32 Nitrogen/Triethylamine Mixed-Gas Separation Properties

Hyflon® AD60 membranes were prepared as in Example 11, and membranestamps were subjected to permeation experiments using the same generalprocedure as in Example 1. The temperature was 21° C., the pressure was65 psia, and the feed gas mixture contained triethylamine (TEA) invarying concentrations from 0.7-1.9% (1.9% is saturation) and thebalance nitrogen. The pressure-normalized fluxes of TEA and nitrogenwere measured, and the nitrogen/TEA selectivities at the varying TEAconcentrations were calculated. The results are shown in Table 22.

TABLE 22 TEA Mixed-Gas Pressure- N₂/TEA Concentration Normalized Flux(GPU) Selectivity (%) N₂ TEA (−) 0.7 17.5 0.08 220 1.6 16.6 0.11 151 1.716.2 0.28 58 1.9 15.8 0.25 63

FIGS. 34 and 35 are graphs showing the measured pressure-normalizedfluxes and the calculated selectivities, respectively. As can be seen inFIG. 34, the nitrogen flux remained nearly constant over the range ofTEA concentrations; the TEA flux increased as the TEA concentrationincreased. As a result, the nitrogen/TEA selectivity decreased as theTEA concentration increased, as shown in FIG. 35. Again, the membranesretained acceptable flux and selectivity even in the presence of TEA atsaturation.

Example 33 Comparative Example with Polyimide Membrane

A polyimide membrane (BPDA-TMPD) was prepared as in Example 29, andmembrane stamps were subjected to permeation experiments using the samegeneral procedure as in Example 1. The temperature was 22° C., thepressure was 65 psia, and the feed gas mixture contained 1.6%triethylamine (TEA) and 98.4% nitrogen. The pressure-normalized fluxesof TEA and nitrogen were measured, and the nitrogen/TEA selectivity wascalculated. The results are shown in Table 23.

TABLE 23 Mixed-Gas Pressure-Normalized Flux (GPU) N₂/TEA Selectivity N₂TEA (−) 6.2 610 0.01

As can be seen, the polyimide membrane is clearly TEA-selective, incontrast to the membranes of the invention, which maintain usefulnitrogen/TEA selectivities throughout the range of TEA concentrations.

Examples 34-36 Process Designs Example 34 Hydrogen Recovery

A computer calculation was performed with a modeling program, ChemCad V(ChemStations, Inc., Houston, Tex.), to illustrate the process of theinvention as reflected in the recovery of hydrogen from refinery off-gasdestined for the fuel header. The process was assumed to be carried outas shown in FIG. 36. Referring to this figure, refinery off-gas stream201 at 200 psia passes to compressor 202 where it is compressed to 400psia, stream 203. After passing through the compressor aftercooler, 204,the gas is passed as feed stream 205 to membrane separation unit 206.The membrane separation unit was assumed to contain membranes, 207,providing gas fluxes consistent with the membranes taught in thedetailed description of the invention, for example, Hyflon® D AD60.

The flow rate of the raw off-gas was assumed to be 5 MMscfd, and the gaswas assumed to contain 35% hydrogen, 5% nitrogen and 60% C₁-C₆hydrocarbons, of which 15% were assumed to be C₃₊ hydrocarbons. The rawgas was assumed to be at 200 psia and 33° C. The permeate side of themembrane was assumed to be at 20 psia. The results of the calculationsare summarized in Table 24.

TABLE 24 Stream 201 205 208 212 213 Flow (MMscfd) 5 5 1.4 1.4 3.6Pressure (psia) 200 400 20 200 400 Temperature (° C.) 33 60 60 40 63Dewpoint (° C.) 33 49 −76 −49 58 Component (vol %): Hydrogen 35 35 90.090.0 13.6 Methane 30 30 6.5 6.5 39.1 Ethane 15 15 0.4 0.4 20.7 Propane10 10 0.2 0.2 13.8 n-Butane 3 3 <0.1 <0.1 4.2 n-Hexane 2 2 <0.1 <0.1 2.8Nitrogen 5 5 2.9 2.9 5.8

The hydrogen-rich permeate stream, 208, is withdrawn from the membraneunit and passes to compressor 209, where it is recompressed to 200 psia,stream 210. After passing through the compressor aftercooler, 211, thehydrogen product stream emerges as stream 212 for use as a hydrogensource in the refinery. Obviously, if the hydrogen were not needed atpressure, the second compressor could be omitted. The residue stream,213, now at close to its dewpoint, is withdrawn from the feed side ofthe membrane unit. This stream is reduced in volume from 5 MMscfd to 3.6MMscfd and in hydrogen content from 35% to 14%, and would be suitablefor sending to the fuel header. The process of the invention recoversabout 70% of the hydrogen originally in the raw off-gas in reusableform.

Example 35 Carbon Dioxide Removal/Hydrocarbon Recovery

A computer calculation was performed with a modeling program, ChemCad V(ChemStations, Inc., Houston, Tex.), to illustrate the process of theinvention for the recovery of carbon dioxide, pipeline gas, and naturalgas liquids from associated gas produced by oilfield flood operations.It was assumed that the process involved treatment of the raw associatedgas by Hyflon® AD60 membranes, followed by treatment of the remaininghydrocarbon-rich gas by membranes selective for the heavier hydrocarbonsover methane. In this way, the process was able to deliver three productstreams: a carbon dioxide stream suitable for reinjection into theformation; a natural gas liquids (NGL) stream; and a light methane-richstream, containing only 4% carbon dioxide, suitable for acceptance intoa natural gas pipeline. The flow rate of the raw associated gas wasassumed to be 20 MMscfd, and the gas was assumed to be of the followingcomposition:

Carbon Dioxide 60.0% Methane 23.5% Ethane 7.0% Propane 6.0% n-Butane3.0% n-Pentane 0.5%

The process was assumed to be carried out as shown in FIG. 37. Referringto this figure, gas stream 301 at 415 psia is passed as the feed streamto the first membrane separation unit, 319, which was assumed to containmembranes as in Example 34. Carbon dioxide permeates the membranepreferentially to produce permeate stream 303, which contains almost 97%carbon dioxide and is suitable for reinjection. As a result of removalof carbon dioxide, the first residue stream, 302, is enriched inhydrocarbons, thereby taking the hydrocarbon content beyond the dewpointand creating a two-phase mixture. Stream 302 is mixed with thelight-hydrocarbon-enriched off-gas, stream 315, from separator 325. Themixed stream, 304, is passed to the first phase separator, 320, fromwhich is withdrawn a small liquid hydrocarbon stream, 306. The separatoroverhead stream, 305, is passed to the second membrane separation unit,321, which was assumed to contain the same membranes as in membrane unit319. The second residue stream, 307, is passed to the second phaseseparator, 322, from which is withdrawn an additional liquid hydrocarbonstream, 310. The second permeate stream, 308, is mixed with firstpermeate stream, 303, to form carbon dioxide-enriched stream 309 forreinjection. The second separator overhead stream, 311, is passed to thethird membrane separation unit, 323, which was assumed to containsilicone rubber membranes. Methane-enriched residue stream 312 may bepassed to the pipeline directly or after additional treatment. Permeatestream 313 is mixed with overhead stream 317 and recompressed incompressor 324. This stream is mixed with C₂₊ -hydrocarbon-enrichedstreams 306 and 310, and passed as stream 314 to the third separator,325. The separator overhead stream, 315, is recirculated to the firstresidue stream for additional hydrocarbon recovery. The C₂₊-hydrocarbon-enriched bottoms stream, 316, is lowered in pressurethrough valve 327 and passed to the fourth separator, 326, from which iswithdrawn a natural gas liquids product stream, 318. The separatoroverhead stream, 317, is mixed with the third permeate stream foradditional hydrocarbon recovery.

The results of the calculations are shown in Table 25.

TABLE 25 Stream 301 302 303 304 305 306 307 308 309 310 311 312 313 314315 316 317 318 Gas Flow (MMscfd) 20 15.2 4.8 20.6 20.6 — 12.3 8.3 13.1— 11.3 5.4 5.9 — 5.4 — 1.1 — Liquid Flow (bpsd) — — — — — 17.5 — — — 667— — — 1,634 — 1,687 — 1,070 Flow (lbmol/h) 2,196 1,668 528 2,262 2,2593.0 1,352 907 1,435 112 1,241 591 650 884 594 289 119 170 Pressure(psia) 415 415 20 415 415 415 415 20 20 415 415 415 20 415 415 415 20 20Temperature (° F.) 68 58 63 59 59 59 66 62 63 66 66 22 44 66 63 63 −62−62 Component (mol %): Carbon Dioxide 60.0 48.4 96.7 39.5 39.5 17.3 8.286.2 90.0 3.4 8.6 4.0 12.8 11.6 14.4 6.0 13.0 1.1 Methane 23.5 30.1 2.632.2 32.2 6.4 47.1 9.9 7.2 9.9 50.5 69.7 33.0 28.0 37.9 7.8 18.6 0.3Ethane 7.0 9.1 0.5 14.4 14.4 12.6 22.2 2.7 1.9 18.8 22.5 17.9 26.5 27.829.2 25.0 43.3 12.2 Propane 6.0 7.8 0.2 9.8 9.8 25.4 15.7 0.9 0.7 34.014.1 7.0 20.5 22.5 15.3 37.3 22.8 47.5 n-Butane 3.0 3.9 0.1 3.7 3.7 28.06.0 0.2 0.2 27.7 4.0 1.3 6.5 8.6 3.0 20.2 2.2 32.9 n-Pentane 0.5 0.7 ---0.5 0.5 10.3 0.9 --- --- 6.3 0.4 0.1 0.7 1.3 0.2 3.6 0.1 6.0 --- = lessthan 0.1 Membrane area = 1,000 + 4,350 m² (Hyflon ® AD60) + 419 m²(Silicone rubber) Horsepower requirement (theoretical) = 1,092 hp

The 20 MMscfd of raw associated gas entering the system yields 13.1MMscfd of carbon dioxide for reinjection, and 5.4 MMscfd ofpipeline-quality natural gas. In addition, nearly 1,100 barrels per day(bpsd) of natural gas liquids (stream 318) are recovered.

Example 36 Olefin/Paraffin Separation

A computer calculation was performed with a modeling program, ChemCad V(ChemStations, Inc., Houston, Tex.), to illustrate the process of theinvention for the separation of olefin/paraffin mixtures as might benecessary in a petrochemical manufacturing plant. The stream to betreated was assumed to contain 80% propylene and 20% propane. Themembrane separation process was assumed to be carried out in a singlestage as shown in FIG. 1. The membrane, 3, was assumed to be as inExample 34. Stream 1 is the feedstream, stream 4 is thepropylene-depleted residue, and stream 5 is the propylene-enrichedpermeate, which may be recompressed if necessary and recycled to themanufacturing process. The feed gas was assumed to be at 150 psia and25° C. The results of the calculations are summarized in Table 26.

TABLE 26 Stream 1 4 5 Mass Flow (lb/h) 3,724 540 3,184 Pressure (psia)150 150 15 Temperature (° C.) 25 24 25 Component (lb/h) Propane 773 307467 Propylene 2,951 234 2,717 Component (mol %): Propane 20.0 55.6 14.1Propylene 80.0 44.4 85.9

Membrane area=2,598 m²

We claim:
 1. A process for separating a desired gas from a gas mixturecomprising the desired gas and a C₃₊ hydrocarbon vapor, the processcomprising the steps of: (a) passing the gas mixture across the feedside of a separation membrane having a feed side and a permeate side,the separation membrane having a selective layer comprising a polymerhaving: (i) a ratio of fluorine to carbon atoms in the polymer greaterthan 1:1; (ii) a fractional free volume no greater than about 0.3; and(iii) a glass transition temperature of at least about 100° C.; and theseparation membrane being characterized by a post-exposure selectivityfor the desired gas over the C₃₊ hydrocarbon vapor, after exposure ofthe separation membrane to liquid toluene and subsequent drying, that isat least about 65% of a pre-exposure selectivity for the desired gasover the C₃₊ hydrocarbon vapor, as measured pre- and post-exposure witha test gas mixture of the same composition and under like conditions;(b) providing a driving force for transmembrane permeation; (c)withdrawing from the permeate side a permeate stream enriched in thedesired gas compared to the gas mixture; (d) withdrawing from the feedside a residue stream depleted in the desired gas compared to the gasmixture.
 2. The process of claim 1, wherein the desired gas is chosenfrom the group consisting of hydrogen, nitrogen, oxygen, argon, carbondioxide and organic compounds.
 3. The process of claim 1, wherein thedesired gas is hydrogen.
 4. The process of claim 1, wherein the desiredgas is nitrogen.
 5. The process of claim 1, wherein the desired gas ismethane.
 6. The process of claim 1, wherein the desired gas is carbondioxide.
 7. The process of claim 1, wherein the desired gas ispropylene.
 8. The process of claim 1, wherein the C₃₊ hydrocarbon vaporcomprises a paraffin.
 9. The process of claim 1, wherein the C₃₊hydrocarbon vapor comprises an olefin.
 10. The process of claim 1,wherein the C₃₊ hydrocarbon vapor comprises an aromatic compound. 11.The process of claim 1, wherein the C₃₊ hydrocarbon vapor is chosen fromthe group consisting of halogenated compounds, amines, ketones andalcohols.
 12. The process of claim 1, wherein the gas mixture furthercomprises methane.
 13. The process of claim 1, wherein the desired gasis nitrogen and the gas mixture further comprises methane.
 14. Theprocess of claim 1, wherein the selective layer comprises aperfluorinated polymer.
 15. The process of claim 1, wherein theselective layer comprises a polymer formed from a fluorinated dioxolemonomer.
 16. The process of claim 1, wherein the selective layercomprises a polymer formed from a fluorinated dioxolane monomer.
 17. Theprocess of claim 1, wherein the selective layer comprises a polymerformed from a fluorinated cyclically polymerizable alkyl ether.
 18. Theprocess of claim 1, wherein the selective layer comprises aperfluorinated polyimide.
 19. The process of claim 1, wherein theselective layer comprises a copolymer.
 20. The process of claim 1,wherein the selective layer comprises a copolymer formed fromfluorinated dioxole and tetrafluoroethylene repeat units.
 21. Theprocess of claim 1, wherein the polymer has the formula:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.
 22. The process of claim 1,wherein the selective layer comprises a polymer having the formula:

where n is a positive integer.
 23. The process of claim 1, wherein theseparation membrane is an integral asymmetric membrane.
 24. The processof claim 1, wherein the separation membrane is a composite membranecomprising the selective layer supported on a microporous supportmembrane.
 25. The process of claim 1, wherein the separation membranecomprises a composite membrane that comprises a support membrane, agutter layer and the selective layer, the gutter layer forming a layerbetween the support membrane and the selective layer.
 26. The process ofclaim 25, wherein the gutter layer comprises a copolymer ofperfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene.
 27. Theprocess of claim 1, wherein the gas mixture, as brought into contactwith the feed side, has a total C₃₊ hydrocarbons partial pressure of atleast about 25 psia.
 28. The process of claim 1, wherein the gas mixturecontains at least about 5% C₃₊ hydrocarbons.
 29. The process of claim 1,wherein the gas mixture, as brought into contact with the feed side, hasa pressure of at least about 30% of the saturation vapor pressure of thegas mixture.
 30. The process of claim 1, wherein the post-exposureselectivity is at least about 70% of the pre-exposure selectivity.
 31. Aprocess for separating hydrogen from a gas mixture comprising hydrogenand a C₃₊ hydrocarbon vapor, the process comprising the steps of: (a)passing the gas mixture across the feed side of a separation membranehaving a feed side and a permeate side, the separation membrane having aselective layer comprising a polymer having: (i) a ratio of fluorine tocarbon atoms in the polymer greater than 1:1; (ii) a fractional freevolume no greater than about 0.3; and (iii) a glass transitiontemperature of at least about 100° C.; and the separation membrane beingcharacterized by a post-exposure selectivity for hydrogen over the C₃₊hydrocarbon vapor, after exposure of the separation membrane to liquidtoluene and subsequent drying, that is at least about 65% of apre-exposure selectivity for hydrogen over the C₃₊ hydrocarbon vapor, asmeasured pre- and post-exposure with a test gas mixture of the samecomposition and under like conditions; (b) providing a driving force fortransmembrane permeation; (c) withdrawing from the permeate side apermeate stream enriched in hydrogen compared to the gas mixture; (d)withdrawing from the feed side a residue stream depleted in hydrogencompared to the gas mixture.
 32. The process of claim 31, wherein theselective layer comprises a polymer formed from a fluorinated monomerchosen from the group consisting of dioxoles, dioxolanes, alkyl ethersand perfluorinated polyimides.
 33. The process of claim 31, wherein theselective layer comprises a copolymer formed from fluorinated dioxoleand tetrafluoroethylene repeat units.
 34. The process of claim 31,wherein the polymer has the formula:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.
 35. The process of claim31, wherein the separation membrane comprises the selective layersupported on a microporous support membrane.
 36. The process of claim31, wherein the gas mixture further comprises methane.
 37. The processof claim 31, wherein the separation membrane exhibits a mixed-gasselectivity for hydrogen over methane as measured at the operatingconditions of the process of at least about
 10. 38. The process of claim31, wherein the separation membrane exhibits a mixed-gas selectivity forhydrogen over propane as measured at the operating conditions of theprocess of at least about
 50. 39. The process of claim 31, wherein thegas mixture is a refinery off-gas stream.
 40. The process of claim 31,wherein the post-exposure selectivity is at least about 70% of thepre-exposure selectivity.
 41. A process for separating propylene from agas mixture comprising propylene and propane, the process comprising thesteps of: (a) passing a gas mixture comprising propylene and propaneacross the feed side of a separation membrane having a feed side and apermeate side, the separation membrane having a selective layercomprising a polymer having: (i) a ratio of fluorine to carbon atoms inthe polymer greater than 1:1; (ii) a fractional free volume no greaterthan about 0.3; and (iii) a glass transition temperature of at leastabout 100° C.; and the separation membrane being characterized by apost-exposure selectivity for propylene over propane, after exposure ofthe separation membrane to liquid toluene and subsequent drying, that isat least about 65% of a pre-exposure selectivity for propylene overpropane as measured pre- and post-exposure with a test gas mixture ofthe same composition and under like conditions; (b) providing a drivingforce for transmembrane permeation; (c) withdrawing from the permeateside a permeate stream enriched in propylene compared to the gasmixture; (d) withdrawing from the feed side a residue stream depleted inpropylene compared to the gas mixture.
 42. The process of claim 41,wherein the selective layer comprises a polymer formed from afluorinated monomer chosen from the group consisting of dioxoles,dioxolanes, alkyl ethers and perfluorinated polyimides.
 43. The processof claim 41, wherein the selective layer comprises a copolymer formedfrom fluorinated dioxole and tetrafluoroethylene repeat units.
 44. Theprocess of claim 41, wherein the polymer has the formula:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.
 45. The process of claim41, wherein the separation membrane comprises the selective layersupported on a microporous support membrane.
 46. The process of claim41, wherein the separation membrane exhibits a mixed-gas selectivity forpropylene over propane as measured at the operating conditions of theprocess of at least about 2.5.
 47. The process of claim 41, wherein thepost-exposure selectivity is at least about 70% of the pre-exposureselectivity.